Someone I know wants to sell me a tube amp. I know very little to nothing about tubes, since I haven't played through one. He says the problem with it is that you play it for a little while and it sounds great. After 15-20 minutes, it sounds like crap. After letting it rest, it will do the same thing again, good for a little bit, then crap again. Anyone have any ideas? Is it an easy fix? Does it just need new tubes? Thanks.