Shutting down my system tonight, I noticed a slight hum coming through my new subwoofer. It isn't noticable when there's music or video playing, but when the room is quiet, there's a definite hum. Is this normal? Anything I can do to eliminate it? Right now, my A/V equipment is running on its own dedicated 120VAC circuit, and the sub is fed using a quality RCA cable from the LFE port on the receiver. Thanks for your help.