The hum might be caused by a component or transformer near your preamp/processor. I had a small 12V transformer slip under my lexicon processor and it created a hum just as you describe. Solution...move the transformer (or other device that causes the hum).
HELP: how to eliminate speaker HUM?
I've developed a nasty, fairly loud hum from all five speakers in my HT setup as well as my sub. If I turn off amp, all humming stops. If I disconnect IC from preamp, that channel/spkr hum stops. Is this ground loop hum? (I've heard of this but don't know what it means). I've heard there may be devices that stop the hum ("humbusters"?). Please help. Thanks..Jeff