Not amplifying

I've narrowed the problem down to the right channel tube mono-block. Music is coming out of the speaker but at a low level. The bias of the tubes is correct and holding steady. I opened up the chassis and don't see any fried connections or leaking caps. The left channel is working fine. Any suggestions?
Listing your system and what you did to narrow the problem down to one of the amps, would probably be a good place to start.
It turned out to be a bad pre-driver tube. Thx.
Same thing happened to me once, and the driver tube was the last thing I suspected :(