I lent my 87 SVT + 8x10 to someone. Ever since, I now get crazy feedback when I turn either the bass or treble knobs more than 3/4 of the way up. Anyone got any ideas what could be up with it? (I live pretty far from any decent tech, so I thought I'd ask here first). I've tried different cables etc- still happens. Also, when I got my amp back, the impedance switch was set to 2 ohms instead of 4, where it should have been. Would this be a possible cause of any damage? thanks..