I'm not sure what causes it--all amplifiers hum (some more than others, of course)--and I think it's inevitable that you hear that through the speakers, especially though more sensitive speakers. I have 96dB effcient speakers with 1,000W built-in subwoofers, and I can hear the hum from the subs' tranformers if I get close enough. Fortunately, I generally can't hear it from my seat.
