Scratch that. Was referring to the preamp valve problem comment. What would that cause?

Thanks for the awesome post Andy. One quick question. You mentioned a bad tube in the pre amp section. What would be happening if that were the case? Would switching this out just to see what happens be worth it in your opinion?

I currently have a Terror Bass 500 head and an SP410 cabinet. I run a rickenbacker 4003 to the amp. I have noticed at rehearsal that as I turn up, the amp begins to get "farty" for lack of a better term. After asking around, people have suggested I boost mids and cut bass and treble a bit. They also...