Dragon, it still appears to be a problem with the internal sub amplifier, despite the fact that it's been replaced already. With nothing connected to the sub but the power cord it can't be a ground loop hum because there's nothing to form a loop. The rectifier and filter capacitors in the power supply section of the sub amp are supposed to supply clean DC, with inaudible hum, to the amplifying section.

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Enjoy the music, not the equipment.