Charles, I don't really have any new suggestions, since if the speakers hum with the A1400-8 powering them, but not with the Onkyo receiver, there's no ground loop question, but as you suspect, an amplifier problem. The power supply section of amplifiers, in particular the rectifier and filter capacitors, is supposed to provide clean DC free of audible hum when operating correctly. When you sent it in for repair of the dead output channels did you also describe the hum problem?

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Enjoy the music, not the equipment.