I have a JBL PowerBass PB 12 subwoofer incorporated into my home theatre system. The sub has worked wonderfully. Yesterday, however, I came home only to hear a lot of loud static coming through the sub. This, even though the reciever was off. The only way I could stop the static was to either flip the power switch to "off" or unplug the speaker from the wall outlet. What gives here? Has the amplifier gone bad? What to do???