So I bought a used 6505+ 112 combo last night I ordered online. It has this constant crackling noise when turned on but the noise stays at a steady volume I can play over. I opened the back and pulled out one of the power tubes and put it back in just to see how they come out. After that, the amp got real noisy, volume went it and out, and all the gain would suddenly disappear. I swapped cables and wiggled the tube I had messed with before. After a few minutes it went back to before, playable but with same crackling. Would replacing the preamp tubes fix all the problems, or just the noise? When it went intermittent, was that a sign to replace preamp tubes, power tubes, or just clean the sockets? Also, I've read mixed things about rebiasing a 6505+, it is needed? Sorry for the noob questions but this is my first tube amp.