I've seen many of these problems. In most cases it's caused by a ground loop. For instance, most people create a ground loop by having both the DVD player and the TV connected to the amplifier, while the DVD and TV are connected to eachother by SCART.
Also, switched-off equipment can still generate a buzz. Try disconnecting all equipment, only leave one source connected. If the buzz disappears, try connecting all equipment one by one to find the problem.
Sometimes it's simply caused by a bad cable between power amp and processor. This should be easy to check (try different cables).
Good luck,
Johannes Grimmerink