Ground Loop Isolator
If you are getting a low frequency (60 Hz) hum induced into your system and you have identified the audio component causing the problem, you can easily remove the hum by using an RCA patch cable with a built-in "Ground Loop Isolator". Radio Shack has one for under $20: http://www.radioshack.com/product/index.jsp?productId=2062214
When checking for components that might be causing this problem, look first at a computer being used as an audio source as this is a fairly common problem. Then also check for audio devices that are plugged into a circuit different from the one in which the whole-house audio controller and multi-channel amplifier are plugged into.