For those who appreciate a sense of closure in their forum Q&A, here's a follow-up to my previous posts. I got my new pre/pro (Onkyo 885) and hooked it up via XLR cables to the 7500. Almost all of the noise in my speakers is now gone. The 7500 itself still makes some mechanical noise, and a little of that is transferred to the center channel speaker (the speaker closest to the amp, but this could be a coincidence), but entirely liveable. If I had more time, it would be interesting to see if the buzzing was still there if I used unbalanced connections between the Onkyo and the 7500. If not, it would strongly suggest that the unused amps in the receiver I had been using as a preamp were the culprit, which was actually my initial question in my original post. I suspect it was a combination of the Yamaha and the unbalanced connections, but now that I've got it the way I want it, I don't need to fix what ain't broken.

Now I can actually relax and enjoy the wonderful sounds coming from the 7500 and 885. I'm also thinking about trying out one of those PS Audio Humbusters to get rid of that transformer noise. There seems to be mixed reviews on its effectiveness, but I'm interested in giving it a shot. If I do and it works, I'll post back here later.