I've had my amp (with the extra cabinet) at home for years and it's been great to play with. But one day (maybe a year ago), I turned it on and HMMMMMMMMMMMM. Big ground hum. No idea why that would happen. So,, I walked away (I have other amps) and let it gather dust until today. Still the same hum. I'm taking it to the reasonably local service tech in Sacramento tomorrow ... but anyone have an idea what's he's going to find is the problem?