I'm going to write a short book here - sorry ...
RFI will be your biggest concern in an apartment building, and I wouldn't even consider an amplifier.  Even 100 Watts will cause issues, and revealing your operations to your neighbors could bring the management down on you, a good thing to avoid.  You need the highest efficiency antenna possible, and you will be able to use the SGC tuner in many circumstances. I think that is probably your best investment.

If you can get >60' of wire stretched up into the air it will tune 80m with good efficiency and may reach 90% if you can put out a couple of wire radials. Radials can be stretched along the building walls in any arrangement - straightness is not that meaningful.  Burial will hide them, but If you can staple or glue or otherwise fasten the radials to the building wall even a couple of inches above ground level you can get away from a lot of ground loss - a big improvement.  A couple of elevated radials will equal or outperform many buried radials, so this is a good thing to do.  Radials can be run up and over doorways without harming antenna performance much, too. Elevated radials have to be cut to resonance, though, so at least one per band is advisable (though a 33' radial for 40m will also be usable for 15m).

Think creatively.  Getting the antenna away from the building is best from an RFI perspective but you may not be able to do that.  Here are some examples I have run across:
- a long wire of 40 gauge magnet wire (REALLY invisible) tossed over a tree and tuner-fed against a ground rod behind flower beds.  When the wire broke he would pull as much of it out of the tree as possible and toss a new one up - that happened once or twice a year.  Another ham did the same but was on an upper floor with no way to use a ground rod, so he grounded to everything conductive in the area - the balcony railing, the frames around his sliding glass doors and windows, etc.  (Just don't be transmitting above 5 Watts or so if someone has their hand on the window frame!) 
- One ham used an aluminum downspout as his vertical, tuned against a ground rod.  He managed to get up to the gutters with a ladder one day and bonded more sections of the gutter system by putting sheet metal screws between them to ensure a good electrical connection.  Another ham had plastic gutters, but managed to stretch some wire out in the bottom of the gutters and ran his feedline down a handy downspout, recognizing that he might have to re-do it if management ever cleaned the gutters.
- One ham stapled a loop of 22 gauge hookup wire under the eaves of the building, all the way around, and fed it with a short section of ladder line down to his autotuner - it worked great.  He chose the color of the wire to blend in, and I believe he painted the ladder line.  While this might affect the impedance of the line, it's not a big hit on performance, and stealth is more important in an apartment situation.
- Other hams have tried tuning the wiring of the building, but this is a great way to create terrible RFI.  Also, ductwork, wiring, and building structure can parasitically soak up RF and kill your antenna's performance, so even getting it outside the wall or (better yet) more than a foot from the structure can make a BIG improvement.  I had an indoor dipole, and then a loop, that worked terribly in my house.  I changed to a 40m dipole thrown over the roof, but it was still terrible.  Then I managed to use a nearby tree to get it just a couple of feet above the roof and I was suddenly making lots of QSO's.  I believe the ridge vent, a metal screen running the length of the roof peak, was killing performance until I got away from it.

There are a bunch of ideas.  Good luck, and I hope to catch you on the air soon.  73 de kt8k - Tim!

I don't understand why only Japanese radios are mentioned.  Maybe it's for the same reason that I see so few Tentecs for sale at swaps or elsewhere - most of us who have them are NOT letting them go.  That ought to tell you something.  (I have 3 TT rigs and a TS-850 that almost never gets powered on.)  I'm not saying Tentecs are "god's gift to radio" or anything, but I've loved their design philosophy and performance ever since I got my first one in 1978.   My contest scores prove they work, too.

Tentec isn't the only great American radio company, though - I don't have an Elecraft radio, but have noticed a LOT of the serious contesters (who need the very best) getting K3's, and I have friends with other Elecraft radios that swear by them.
Just suggesting you look around a bit more ...
Good luck & 73 de kt8k - Tim

Just to elaborate a little on what K1CJS said, I've used regular electronics solder in my outside antennas and found it turned to a white powder as little as 1-2 years later.  Now I use, believe it or not, wire nuts in my wire antennas.  If I want to improve on that I hit them with a good spray paint before they go up in the air.  Those connections are excellent, low resistance, and have proven to be extremely durable. 

For grounding, though, I use only commercial grounding clamps and, as was said above, at least #6 copper.  My lightning arrestors are mounted to a copper plate enclosed in a sizeable weatherproof NEMA enclosure bolted to the house (swap cost $125), a foot above the first ground rod in the system.  Then I put a coaxial choke, right beneath the box, in each feedline going to the radio room so the lightning will see a lower impedance in that 1 foot of copper between the NEMA box and the ground system than in the connections going inside.

It sounds like you are on the right track, BTW.  Good reception & 73!  Tim, KT8K

I, too, have had limited benefit with standard contact cleaners or, at least, the pot was scratchy again within 6 months.  I have had much better results with CAIG Contact Re-Nu and other CAIG products.  You can find and order them on-line.  (I have no relationship with CAIG except to have bought one of their very economical kits, which includes several of their contact cleaners and post-cleaning treatments.)

Alternatively you could replace the pot, if you could find exactly the right one and have the skill and tools needed.  In my experience I have gotten longer life out of pots I cleaned and treated with the CAIG products than I have with new pots. 
Best or luck with your rig, and 73 de kt8k - Tim

I find that insulated wire is affected less by contact with leaves and branches - less loss, especially if you're running QRP, is a good thing.  I don't bother with copperweld or other special wire, and usually use standard hookup or bell wire.  It's cheap and works just fine. 
Good reception & 73 de kt8k - Tim

I've got one of the 33' DK9SQ masts, but it seems far too lightly build for permanent installation.  You would need to add some good clamps to prevent the sections from loosening and telescoping by themselves, which can blow out the metal-ring stops at the bottom of the sections (I speak from experience).  Mine is also quite thin and flexible near the top - too much so to support even a thin wire antenna under much stress.  It wouldn't last the first winter here, that's for sure.  For Field Day it's great, but I'd consider something much more substantial (the wood mast idea sounds good) for permanent or semi-permanent outdoor mounting.
Good luck & 73 de kt8k - Tim

I don't know how much listening you've done in the past year, but hearing only those few states makes me think there is something fundamentally wrong in your antenna system beyond just height.  When faced with an antenna on which I am hearing so little I immediately start checking for bad connectors and similar problems.

I assume you've tried transmitting as well as receiving with this antenna, so I'm curious if you've measured reactance and resistance or at least SWR.  

I had a seemingly-dead antenna that worked like yours, a dipole lying across the roof of the family room.  I raised it several feet into the air above the roof and the airwaves opened up with many, many signals.  Apparently the vent along the peak of the roof included a metal screen, and it must have been inducing horrendous loss.  A few feet of distance reduced its influence tremendously, and suddenly I had a working antenna.  That's the only case I have seen where additional height made the difference between hearing lots of signals and hearing almost none.  In most cases raising an antenna might provide an additional S unit or so, but not "make or break" antenna performance.

I recommend you check your connections (sometimes PL-259 connectors accidentally get assembled with a short or open ...), substitute feedline(s), and let us know some more details about your setup.  Stretch a piece of wire out and stick the end in the female connector center sockets starting at the radio (where you should hear plenty of signals on 20m during the day or 40 or 80m at night). Then work your way out the chain of feed lines and see if you hear a big change at some point that would indicate a bad connection.
Best reception & 73 de kt8k - Tim

How about connecting the Pro64 to ground, too.  Does that make a difference?  Your interference source sounds like it might be common mode on the power or antenna cables, and might be choked off or reduced with ferrite beads or other means.  I suspect an internal fault, open ground, or design flaw in the Pro64.
I am interested in seeing what ideas the experts can come up with.  Please make another entry when you figure it out and let us all know, thanks.
Best of luck de kt8k - Tim

Is the Pro-64 running squelch, so that noise is only heard when the squelch opens?  
OR, since the noise includes your furnace and hence can't be internally generated, could noise be coming in via a different route such as the power cord, and somehow be finding its way into the receiver IF chain at a point where something that responds to the received signal, such as AGC, is involved?  Try RF-choking the power cord first to check that path.  
This seems like a tough one to me, but those are my first thoughts.
Good luck & 73 de kt8k - Tim

The preamp in the mic stand will work in lieu of an impedance transformer - that's a good reason to keep it.  Just be sure it isn't turned up too high or otherwise causing distortion or overloading the radio's input.  Check your signal as best you can to make sure the frequencies are balanced (as mentioned above).  An audio spectrum analyzer would be ideal for adjusting your audio quality, and there is undoubtedly software to do this on your computer.  

Desk mics, however, can be a pain.  They are heavy to hold up to your mouth, or you can get a sore back leaning over to get close to it - not very ergonomic.  A headset (headphones with boom mic, preferably noise-canceling type) is a much better choice, as the mic-to-mouth distance is fairly constant and the mic position can be adjusted for minimum breath-pop and consistent, best-quality sound.

I ran an unamplified D104 and (more usually) its dynamic cousin, the 10DA, for years.  My recently acquired headset (with dynamic, noise-cancelling boom mic) is getting reports in on-air comparison tests like "the boom mic is night and day better compared with the desk mic".  I have put the old D104 and 10DA on the shelf ...
Best rx & 73 de kt8k - Tim

Shoot ... I know some may squawk at this, but I hooked up my 550+ foot horizontal loop with some 300 Ohm TV twin lead, ran it right through the 4" PVC pipe with all the coax cables, hooked it to an old LDG autotuner, and it works great.  (Look up my contest scores of the past year - they're up in part because of the addition of this antenna.)  

I think the talk of excessive losses in TV twin lead, and in running parallel lines next to metal (for more than a 6 inches or so), are overblown.  I also don't worry about the pattern too much - I alternate with my old 20/40/80m inverted fan-vee and use whichever is best at the moment.  Actually, I think about the vertical radiation angle as much as the azimuth pattern, and always remember that the loop's vertical angle will go down as the frequency goes up.

Certainly try changing the length of the feed line by 10' or so with a jumper, or changing the length of the loop by a similar or larger amount.  Experiment with some of these ideas.  Any of the above could cure your problem and get your loop working just fine on 40m.  
Best rx & 73 de kt8k - Tim

That antenna should do well enough for you, but additional length might improve efficiency a bit more.  I recommend thin wire such as 18 gauge magnet wire or similar, as it will be almost impossible to spot from 35' below.  I also recommend using longer wires, but let the ends hang down 20' or so.  They'll still be very hard to spot and may both help with tuning on the lower bands and add a vertical component to your signal that will lower the effective radiation angle.  I used a doublet like this for a few years and it worked amazingly well.
Best rx & 73 de kt8k - Tim

Thanks, Steve.  I wasn't sure so I put the covers back on.  It had been more than a month since I thought I got all the bugs out, but when I disassembled one of the 15m traps I got another multi-dozen ants in the shack.  Fortunately I had the vacuum at hand ...

Leaving off the covers and coating the assembly sounds like a great idea, but I wonder if the covers don't add some to the overall strength of the element.  I am considering going the acrylic caulk route and just "hermetically" sealing the traps this time. I have the caulk, and the day job has left far too little time for antenna work recently - this might be the last good chance to work on this one as the sun is setting much earlier, I have others to put up, and the big horizontal loop must be fixed in my scarce weekend hours between now and the big contests.

Incidentally, I was able to tap out the trap insulators using an old Cushcraft 1/4 wave 2m whip.  It's pretty stiff, and I was able to slide it up inside the covers and pop the insulators out (1 each on the 10m traps and 2 each on the 15m traps) by tapping the exposed whip end gently but persistently on the concrete patio.  It's good to do this outside, as it's amazing how much crud and crap builds up inside those traps over a decade or two, especially when they're suspended from a tree and ants and other critters are using the ropes as a super highway to the ground.  I certainly know why replacing the coax last spring didn't help - the source of loss was inside the traps.

I may insulate the rope from the antenna this time, too, as I was always concerned about extra loss when the polyester rope was saturated with water or covered in ice and potentially shorting around the traps.  In the past I just tied the rope to the U=bolt at the center of the element and then attached it to the bottom of the dipole with a stainless steel hose clamp to keep the thing roughly vertical.  This also allows me to pull the center point of the antenna up to the branch and let the driven side of the dipole stick up well above the supporting tree branch.  Does anyone think insulating the rope from the dipole will make much difference?

My fan dipole may work great at 60', but when the band is coming in or going out and the low radiation angles predominate this vertical dipole (also fed at about 60') can be more than an S-unit better.  Normally the two are about the same except in a couple of directions favored by the fan dipole.

Thanks again & best rx to all de kt8k - Tim

I have been running the driven element from an old Cushcraft A3 for years by hanging it as a vertical dipole in the tree tops.  It always worked great, though I could see some loss when it touched leaves in the warmer part of the year.  Over the last couple of years it got worse, though.  Signals were no longer as strong as before in comparison with my other antennas (all but one of which it used to beat).  I took it down and disassembled it, and took the traps into the shack to see if I could peek inside or at least clean them out.  When I pulled off the end caps I was surprised by an entire ants nest inside one of them (I ran and tossed the trap out on the patio before they got all over the shack) and the biggest spider nest I'd ever seen in another one (that also was quickly pitched outside).

With the bugs gone, I am now tapping the outer covers off the traps and cleaning out the bug nests and crud inside.  My question is ... what kind of sealing can I do to prevent the bugs getting in again, while still allowing the traps to drain off condensation.  Or should I just seal them up liberally with a good acrylic sealant and not worry about condensation?

I always appreciate the advice of the good folks here, and have learned a ton from many of you. Thanks for your help again.
best rx & 73 de kt8k - Tim

The way you described the change, that you were adjusting the antenna and it suddenly got much worse, made me think a connection was undone, and your description of your reception results makes me think "LOSS", such as if your radials became somehow disconnected.  It also sounds like you may have some strong noise sources in the building.
My vertical installations have been lossy in proximity to other metal, so proximity to the building is probably a general issue, but the sudden worsening of performance would have me checking all connections.   If it had happened over a period of months I would have suspected water in the feedline, but that may not be your problem.

If there is a way to just elevate the vertical on a ten foot TV mast, or a taller push-up mast, or otherwise get it away from the building, you could see a big improvement both in escaping some of the noise and reducing loss.  If you elevate it even 8 feet off the ground You could also reduce the radials to 2 or 3 quarter wavelength radials per band, and make them out of thin magnet wire so they are nearly invisible.  I fooled a very picky neighbor with a setup like this for over ten years.  He came over and walked all over the yard with me, and never saw my 14 radials of 18 gauge magnet wire strung like a fan over my small subdivision yard, just 8 feet above his head.  I couldn't even see them from the kitchen window as little as ten feet away unless there was ice hanging on them.  That antenna would have been better higher, but it was a lot better than when I first had it ground mounted.

Your vertical has the potential to work well for you, but losses to the building can be huge, and may be somewhat band-specific depending on possible resonances in the nearby conductive materials.  Higher is better, and may move you away from objects that are sucking up the RF.

BTW, from my experience a properly tuned vertical over a good counterpoise/radial system will have an impedance of around 20-30 Ohms, so if your vertical gets down to an SWR of just under 2:1 you may be in fine shape.  Too low an SWR means loss problems, as does too wide an SWR bandwidth.  If your vertical is supposed to have a 25 kHz 2:1 SWR bandwidth on 80m and you are seeing 50 kHz or more, or if it is showing a 1:1 SWR, start looking for and eliminating causes of loss.

Best of luck with it.  73 de kt8k - Tim