I bought a used Mesa/Boogie 400+ about a year ago, along with a used SWR Goliath III cabinet. I instantly loved the sound. I had tried both in Guitar Center before i brought them. Anyway, last August, I left my rig at my friends house. He's a guitar player, and he has tube amps of his own. I've known him for years, so I figured my gear was in good hands. Up until then, I had no problems from any of the items up until then. I get word from my friend the next day that something happened to my amp. After a whole lot of prodding, he eventually told me that he accidentally turned it on, while it wasn't plugged into the cabinet, and something blew inside. He said smoke came out when it happened, and it kept blowing fuses. I'm good with circuits, but not tube amps in particular. I opened it up, and I saw what looked like a pair of resistors had blown up. I wasn't confident enough to repair it, so I brought it to guitar center to get it fixed. After some time, I got it back. I was told that some resistors, were replaced, as well as 4 of the power tubes. It seemed to function as good as ever. This was about 6 weeks ago. Last week, I started experiencing a problem with my amp. Suddenly, there was a small 'pop' sound, and a low buzzing coming from from the amp. Soon after that, there was a similar pop sound, and the amp was back to normal. This happened once more last week. But today, its happened half a dozen times. Every time it happens, the tubes get very hot, very fast. Something is seriously wrong. I have a few questions. 1) Does anyone have any suggestions as to what could be wrong? 2) Does it seem like this problem is related to the previous problem? Nothing has happened to the amp between then and now, just normal use. All of the connections are fine. The low buzzing continues even when the volume is all the way down, and the volume does not affect the volume of the buzzing. I really need help. Thanks. If you need any more details about specifics, feel free to ask.