I am still thinking it is a tube issue. Did you check the tube you were replacing? are the other channels fine? How about the tube sockets, are the pins making good contact. I had a problem with the tube socket in my formula pre. Same symptoms.
How about the effects loop, is that set to switch with the lead channel? Might want to try it off completely and go from there.
Sorry if you have done all this and I am no help at all, but I would like to help, nothing worse than and amp down.