Jay, if you disconnect enough(of the AV equipment), of course connecting cables as well as power cords, the ground loop has to eventually be broken. I assume that you don't mean that there was still a hum if nothing was connected except the plugged-in Adcom to the speakers with wire.
If you get the cable isolation transformer it should be placed on the main cable feed before any splitters, etc. The idea is to isolate the ground on the cable line(but, as already mentioned, if there's a hum with the cable completely disconnected, that can't be it). Unless the amp has a defective power supply section(apparently not, if a cheater plug stops the hum), it's definitely a connection problem that has nothing to do with filtering of the power supply.
