Well, I bought a used Gallien Krueger 400RB IV head maybe a month ago. I also bought a cab with it (yorkville 4x10, which still works great), and it tested fine. The previous owner had just brought the head back from the same repair shop which I would later take it to. The repair guy felt that he had fixed the problem, and the amp seemed fine for the entire first band practice I used it for. Super loud and amazing tone. The next band practice, it started to make some crackling sounds after a while, almost like someone was adjusting a scratchy volume pot. This sound would continue even when the amp was muted. By the end of the practice, there was a high pitched squealing sound reminiscent of a dial-up internet connection. So, I took the amp to the repair guy, and he looked at it for about two weeks. Said he went over it again and again, checking everything. He seemed determined to figure it out, but in the end said he just couldn't find a problem. Everything looked good. He feels like he may have isolated it to something in the power amp, but that's all he knows. The description he listed on the receipt when I picked up the amp was "After about 10 minutes, it sounds staticky and then it sounds like a dial up modem". So, is this a lost cause? I'm a university student, and I can't really afford another head right now, or even an abnormally expensive repair. I can live without the amp, as I am borrowing a friend's SVT-3 pro head, but it's just not the same. I fear that all hope is lost.