From past experience, buzzing transformers are usually an indication of DC on the line. Try a DC blocker on your amp. Kevin at DIYCable carries a good one (I haven't personally tried it) or a simple DIY solution would be the top schematic here...
The rectifier bridge should be 25a or higher and the caps can be 3.3K/35v as they aren't in line.
Ground loops are generally heard through the speakers.
Credit to Bryston and Occam for the schematic!