Hi,
Thanks for all the info and ideas. Although I'm pretty sure I had already tried lifting the grounds on everything but the preamp, I tried it again, but the hum persists. Considering that the speaker will hum even if it is not connected to anything, only plugged in to the wall to power the subamp, it seems unlikely/impossible to be a ground loop associated with the rest of the equipment.
I have been researching the subpanel/isolation transformer idea for a month or two now and managed to pick up a 5Kva Topaz transformer on the cheap, so I'll probably go this route at some point in the near future.
I suppose I'll need to call the utility co. and ask them to replace the transformer leading to my house. Is this something I'm likely to meet resistance with?
With respect to Gs5556's response, What does EMI/RFI sound like? Kind of naive question I guess, but does it sound like 60Hz hum? What differentiates it? Second, I do have cable and I remember the installation guys hooking up some kind of grounding to my electrical panel, but I think it's mounted to the outside of the box. There is no cable hooked up to any part of the stereo however. And last, I did actually disconnect the water pipe ground a while back to see if it would make any difference, but it didn't. I never tried disconnecting the other ground though while leaving the water pipe connected so I'll give that a try.
I really appreciate everybody's help!
Matt