I agree with you completely about ground loops. That's the most common cause, and (ironically) it seems to affect expensive separates more than inexpensive integrated receivers and amps. The reason is obvious, there are more separate grounds to tie to a common point without developing a loop. Seems like cable TV is a common culprit too, so if the cable is hooked in to the home theater, it almost always is a good idea to run the 75 ohm coax in through an RF transformer. That will knock off the hum in lots of cases.

I always thought that AC driven filaments would be a source of hum. I would have thought the cloud of electrons stays relatively constant but there certainly must be a small heatup/cooldown cycle from AC-driven heaters. I also would have thought the filament line would be a source of inductive coupling into the relatively high impedance signal lines right nearby. But I'm surely not an expert in tube circuits, and I'm really just kicking ideas around, sort of thinking out loud. I'm definitely soaking up the ideas around here, so thanks for your input.