Could be a ground loop hum. I've struggled some with that myself with my 750 -- the refrigerator was on the same circuit as the amp for a long time, and it and the cable TV have conspired from time to time to give me some trouble. Some people recommend lifting the ground on the amp with a cheater plug, but that's probably best left as a last resort and/or temporary solution as it is not the safest thing to do.

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