I've read a number of the threads about Ground Loop problems and have tried to isolate the problem but haven't had any success.

The hum only occurs after the sub is pushed a bit. I can play at normal, let's call it wife, volume, with no issues. When I turn it up it starts a low pitched hum, which stays even after the volume is turned back down to lower levels. I need to shut everything down and start again to make it go away completely. However, while it is humming if I mute the receiver the hum stops until I unmute it, so the hum is coming through the receiver.

The receiver is an HK AVR220 and the sub is a Jamo A3SUB.3. Does the fact that the hum disappears when the receiver is muted indicate a problem with the HK? That wouldn't be a bad thing as it is about to be replaced, however, a problem with the sub would be a big pain.

Thanks very much for any insight that anyone might be able to offer.

Have you grounded your receiver? Feedback loop? How close is the sub to your receiver?