The rule for "no splatter" is very simple:
. . . Adjust the TX audio drive, using the computer-resident software and/or the interface, so the rig shows _no_ ALC action on transmit.
That works for just about every rig, and every soundcard / interface.
And -- until you have things set up and stable -- _ask_ whether you have any visible sidebands on PSK31. The answer should be "no".
This is NOT the way to adjust for the K3. If you do, you'll end up with receiver ALC pumping as it tries to adjust to a signal that is too low. Check out http://www.w7ay.net/site/Technical/K3/Content/digital.html
for the info on properly setting the K3's ALC level.....4 or 5 bars of ALC action is ESSENTIAL for clean signal work.
Computer audio (USB Audio CODEC) should be set for FULL. The "Line" volume should be to a mid-level for initial testing (mine is set at 45). Adjust the audio out on the SignalLink until you see the appropriate amount of ALC action.
The K3 has a VERY clean AFSK mode, if everything is set right.