I had the exact same problem with two different Amps (Rotel RMB-1095 and Parasound Halo A51). I tried just about everything under the sun to stop the amp from buzzing. Just for the record, this was not a ground loop issue. I had no buzzing coming from my speakers.
This is what completely silenced the problem I was having.
http://www.psaudio.com/products/upchb.asp
I was quite skeptical at first but the company told me they would take it back and pay the return shipping if it didn't solve my problem. It did solve my problem and needless to say, I still have the UPC-200HB and haven't looked back since.
The problem in my case was related to noise on the power line. I live in the boonies up on a mountain in the woods. Most of the few neighbors that I have own shops with large power hungry/noisy equipment like lathes, saws, and welding machines. I think all this equipment creates noise on the line and caused my amp to buzz.
But I'm happy to say.... Not any more!