Hi everyone, thanks for the replies! Here's an update, and some answers to your questions:
Nsgarch: I agree, I believe it's dirty power. A voltage check shows 119-122VAC, so it's not due to low voltage. I'm hoping to avoid buying a regenerator, as those are very expensive, and if they need to handle high current (power amps), they're even more expensive. And I have multiple amps! :-( Thanks for the tip on the Exactpower, btw. I hadn't seen that unit before.
Tarsando: No, the old equipment buzzed too, but each item has the problem to a different degree. Some are worse than others. Bad news is, I used to be an electrician, lol... I've already checked for the obvious stuff. ;-)
Restock: I already tried what you describe - shutting off all circuits except the one feeding the system. Same problem. I too was hoping it was something in the house causing it, which I could replace or remove. I may need to contact the power company. :-(
Jameswei: I agree. See above comment.
Dbtrucks: I don't have cable TV, just a DVD video source. No coax of any kind connected to the system. (yeah, I'm weird, lol...)
Joefama: First noticed it with the Polk SRT powered sub amps. They were the first item it was loud enough to hear from across a quiet room. When I upgraded from Adcom to Sunfire pre/pro, it got worse, probably due to the larger internal transformers in the Sunfire stuff. The latest is the plasma which I just bought last month (although that mostly buzzes when turned on.)
Adasilva: In my case, the cheater plug made zero difference. I'm hoping to find a better solution than physically opening up the equipment and damping the offending component (see update below).
Freemand: See update below.
UPDATE: I found a product by PS Audio that claims to cure this exact problem. It's not cheap, however. And the original product is no longer manufactured or sold. The new, replacement product is 2/3 more expensive ($499 vs the original $299, MSRP). Click links below for details. I couldn't locate the old product anywhere, but I found a place that sells the new one for about 25% off list price. I ordered one, it will arrive today, I should know if it works by tomorrow. I'll post my findings here. If this unit doesn't fix it, I will definitely call the power company next. A regeneration unit is the last resort due to the excessive cost ($1-3k).
PS Audio Humbuster (was $299, but no longer available):
http://www.psaudio.com/products/humbustermoreinfo.asp
PS Audio UPC-200HB with Humbuster circuitry ($499 MSRP):
http://www.psaudio.com/products/upchb.asp
PS Audio Premier power regenerator ($2195 MSRP):
http://www.psaudio.com/products/premier_power_plant.asp
