I have a subwoofer hum that I can't figure out. The subwoofer is powered. When connected to the Amp the hum is minimal. When the amp is turned off, the hum gets much louder. If I unplug the speaker cable from the back of the subwoofer the hum disappears. If I unplug the speaker cable from the back of the amp the hum does not change. This is annoying because it forces me to turn off the subwoofer whenever I turn off the amp. Any suggestions?