I recently moved into an apt. and noticed that everytime I turn on my amp it send a buzz through the speakers. Currently, I am not using rear speakers and the 7500 is bi-amping the center and powering my LSi9's. I should have more than enough power. I unplugged the amp and tried running it on a circuit seperate from the one shared by my other components. It still did it. I know the standard pocedure is switching out cables and amps (I have two unsused 2200's) to find the culprate, but I am a very lazy man.

Check for devices elsewhere in the apartment that could be causing a problem - particularly dimmer switches.

I had the same problem and the cause was coming from ground plug. Try plugging the 7500 into a 2 plug converter (you can pick it at any hardware store) but don't ground the wire. This eliminated the buzz and I never had the problem again.

Also I suggest plugging the 7500 and all your equipment into some sort of power conditioner or surge protector in order to protect your equipment.