I have an amp that sounds fantastic when it is working properly. It has a 175 watt, 8" speaker. A few days after I got it, I noticed a midrange hum that varied in volume from hardly noticeable to quite loud. I took it to a local repair shop and tried to replicate it for the technician, but it disappeared when tested in the shop. (Much like the old proverb about going to a mechanic with an automoblile problem -- it goes away when the mechanic is there). Anyway, they kept it for a week and the head guy at the shop had a close look. He called, said the amp sounded amazing and he could find no problems. I came and picked up the amp, thinking maybe it was just one of those things. I took it home and played it for an hour or two. The problem appeared to have resolved it self. Then suddenly I hit an open G string (at practice volume) and mwaaahhhhh. A completely new hum. Much more noticeable and only on the G. Different from the last hum which was most noticeable on the E and A strings. I shut the amp off. I was busy so I could not make an MP3 recording of it immediately. I came back a few days later with my H2 Zoom recorder to document the hum and...it's gone again. The amp sounds great. I have been a musician for a long time and have owned many amps. For the first round of mid range hums I hooked it up as an extension cab to another practice amp, same hum, so I think I have ruled out a problem with the amp's electronics. When it's behaving it sounds great at all volumes, so I cannot see how it could be blown. If someone can please give me an idea of what may be wrong with it I would appreciate it.