When you try to play music do you hear music AND the hum or just the hum? Are there any video sources hooked up to the system (cable/satellite box or TV which is hooked up to a cable/satellite box/external antenna or DVD which is hooked to a TV which is hooked to a cable/satellite/external antenna or a radio tuner which is hooked to an external antenna) Many times (but not always) hum in an amp is caused by capacitors that are nearing the end of their life. The next step in the diagnostic process for me would be to try the amp in another system and if that system starts to hum then it's CERTAINLY the source of the issue.

Do you mean a mechcanical humming from the amplifier, or a humming sound from the speakers? First thing to do is to call Leonard at Audio-Research. They have the best customer service bar-none!