I rehearsed last week and all was well...come this week after not even having touched the settings on my amp because I didn't play it since, I turn it on and straight away it starts howling at the moon (if it could see it anyway). There was just uncontrollable feedback everywhere, even when on the clean channel with a light gain from a pedal. I also swapped guitars to make sure it wasn't them and nope, same thing...all with humbuckers mind you (one JB and one BBpro). I couldn't even silence it by playing or muting the strings, it just went bananas and I didn't even have the gain cranked as I never do anymore. This was on my ENGL Savage 120 by the way. What could cause this? My guess is a tube that's gone bad/microphonic...the question is though would it be a preamp or poweramp tube, and can I pick it just by looking closely at them? I didn't notice any strong glowing or anything.