I recently picked up a Soundcraftsmen 300x4 and when I connect it to my electronic crossover, I pick up a hum. It doesn't matter if the crossover is on or not or even plugged in. And, it seems that the more input plugs I insert into the amp (up to 4 possible), it picks up more hum with each plug. I've changed cords, tried cheater plugs, and even ground the chassis together all to no avail. It is not a bad hum (about like a 60's tube Fender), and you don't notice it when music is playing. But the other amps connected to the crossover are dead quiet and you certainly hear it (Altec 604s) when the music stops. Any ideas?