That's a whole lot of money for a simple loop. Granted, they are nicely made, and may include
preamps, but you can build your own for much less than that. (And their description of how a
"magnetic loop" works is rather out of date with modern thinking.)
I don't know why loops seem to be so difficult to comprehend. For a tuned loop, you just make
it big enough to pick up lots of signal without being so large that it is self resonant, or otherwise
outside the range of your tuning capacitor. One ham I visited used three turns of wire around
the house (over the roof and back through the basement) for AM BC receptions with a simple TRF
receiver. Another ham had a large loop on the back of his closet door, allowing him to easily
rotate it while keeping it out of sight when not in use.
I've built some shielded loops, but the stray capacitance to the shield lowers the self-resonant
frequency and reduces the tuning range. I've had better luck building the main loop just
shielding the coupling loop rather than the whole assembly - an easy way to do this is to use
coax cable. For example, my standard loop assembly has 4 or 6 turns of wire inside a loop of
plastic tubing - an even number of turns allows me to ground the center tap in the same housing
as the tuning capacitor. I then add an extra turn of coax for the coupling loop to the receiver
or preamp. Ideally the shield of the coax would be grounded at each end and open in the
center, but often I get lazy and just ground the shield at the opposite end to where the
center conductor is grounded.
For an untuned loop the common method is to run the ends of a loop (sized for the desired
frequency range) into a balanced amplifier. There was a design in RadCom using a pair
of 2N5109 transistors for a low noise, high dynamic range amplifier with the loop connected
between the bases of the push-pull transistors.