After coming back with the 550X from GC (factory sealed box, $499.99),I was looking online for local ads selling SWR cabs for it. Didn't find any, but saw an ad for a Basic Black that "needed some work" for $80. Went to see it and was pretty amazed. It was dirty, but in very, very nice shape. I asked about the work, and the guy said that sometimes it made "screechy sounds." He took out his Jazz Bass and slapped, snapped, popped and jived and couldn't make it malfunction. The closest we could come was getting crunchy sounds when kicking the cab a bit. The original speaker and horn were fine. He said he thought it was an input jack or a bad preamp tube. He said he bought it for about the same price from a guy thinking it was 100% but after having it screech a couple of times, he just put it in the garage where it's collected dust and doghair ever since. He was cleaning the place out and wanted to get rid of some of his marginal gear. Whatever's wrong with this is probably pretty trivial. Any thoughts on where to start the diagnostics? Whatever it is, it doesn't sound that serious, does it?.