I've got a Mesa 400+ head that I recently had all the power tubes replaced and biased in, but I'm still having some issues. The head will just cut out and not give me the volume I need! I usually run the pre-amp volume at about 9 and the master at like 2 1/2. The last show I played it just went quiet, no blown fuses though. I remedied my problem by switching both volumes to 6 (master and preamp.) anyone have any idea what is going on? I think it might be preamp tubes. Any thoughts y'all?