Well, I received the ground loop isolator today and yes, it did seem to reduce the buzz significantly...but also seemed to choke the volume completely. I could barely hear anything out of the sub when the isolator was in place, where removing it brought back pretty loud bass (but also the very loud buzz).

I took the amp out of bridge mode and that didn't do anything.

The receiver does have a two-prong ungrounded plug but I can't reverse it. Tomorrow I'll try to plug the receiver and amp into another circuit in the house (with a pretty long extension cord) to see if that makes a difference.