Hi Spencer, I'm in the camp of 'you have an RF problem'. I think, based on reading all of the above, that you have more RF susceptibility with with this amp, and that the long cables are somehow able to contribute where the other amps were not so susceptible. I don't think its a sensitivity issue.
There could also be a preamp issue that has not manifested before. If you can get your hands on a DVM, check and see if there is any continuity (less than 1 ohm) between the connectors and the chassis. Sometimes if the chassis is only grounded through the wall and the circuit isn't grounded to it you can run into trouble. That's not so bad in an amplifier but in a preamp it can result in hum, even if it worked fine with other amps!
Back to the RF issue: Have you tried different cables?
There could also be a preamp issue that has not manifested before. If you can get your hands on a DVM, check and see if there is any continuity (less than 1 ohm) between the connectors and the chassis. Sometimes if the chassis is only grounded through the wall and the circuit isn't grounded to it you can run into trouble. That's not so bad in an amplifier but in a preamp it can result in hum, even if it worked fine with other amps!
Back to the RF issue: Have you tried different cables?