I've got two setups here; 7500/990 and an older (but goodie) BK ST202 amp/Conrad Johnson Sonographe pre-amp (for vinyl)....at times, my 7500 is dead silent, other times it hums, likewise, the BK does the same thing. I can almost set my watch for times it will do it....usually around dinner-time, early morning, etc.,i.e., when there is a high demand for power in my area. After trying different outlets, isolating cords, cable, etc, I just write it off as a less than ideal feed from my local power company....it is not annoying enough to give up the 7500's superb performance, or the older BK for that matter.