Jeff,
Sorry to hear you are having a mechanical hum problem with your amp. I have always heard (though I can't say for a fact) that such things *can* be caused by problems on the AC line. They can also be caused by bad (noisy) transformers. Humming transformers in audio gear are a pet peeve of mine. I am amazed at how many manufacturers make entire product lines that hum!
My listening room is also very quiet. The Bryston B14 SST is the quietest amp I have owned -- absolutely inaudible. It is quieter than the power conditioner I have now, quieter than the lamp on my desk, and quieter than the Adcom and Conrad-Johnson amps I owned previously. I am talking now about mechanical hum (though its electrical output is silent when it should be, too).
I've heard a few of these amps elsewhere, and they were all inaudible. Unless there is an AC problem at your house, I think it's quite likely Bryston will be able to fix the problem.