For the loop amplifier/preamp Howard is the reference for questions. I guess that since the PC might be far from the track, and signal is weak and prone to noise, it's a good idea to amplify it as close as possible to the loop before transmitting the long distance to the PC. Also for high frequencies like this there might be 'reflections' if the impedance is not matched. This would apply if all amplification was at the loop and the input was directly the chip pin. Having phantom power on the loop amplifier is a clever technique used in instrument preamplifiers as well when dealing with low level signals (mic)