So why do you think being on a different AC circuit would cause buzzing on your other speakers? Feedback through the coax connection between the sub and AVR? Have you looked at a product called powerbridge? I used several of them so I can located all my power connections in one area. The powerbridge allows you to run romex through the wall and have plugs on each end. I did this so the power conditioner that services my a/v stack of equipment would also service my TV. Check out powerbridgesolutions dot com. I really don't see how putting your sub on one AC circuit and your other components on another circuit would cause AC hum or ground loop from the first circuit to get into the othe speakers. The HTPC could be resonating at a freq that is causing some interference. Turn it off while doing the sweeps and see it goes away.
