A friend of mine has a Super Champ XD, as do I, and he's been having some problems with his, especially at higher volume after it's been in use for a while. It will be playing fine, then all of a sudden develop a loud buzz, that continues even after uplugging the guitar cable. He brought it over yesterday to check out and we noticed a couple things. First the socket where the AC cord plugs in is a bit loose. Also the power tubes were black, compared to mine which were clear. Does that mean they are blown, or are they just different tubes. I'm not that knowlegeable about tubes, but have never seen any like that. I've only replaced tubes for performance, not because they were blown. Anybody have any advice about this?