I have been having problems with my Gibson GA-30rv head for a while now. I've had it fixed a couple times for a couple different problems. The last time it was fixed the note about the repair said something along with the lines of tube sockets were not properly alligned and that it was a manufacture flaw or something. It said that the same issue would probably present itself later. I have just experienced a problem again where the amp powers up but there is no output at all. I opened up the amp and found that when powered up, all 4 power tubes fire up but 1 or 2 of the preamp tubes do not. Should I just try replacing them? Or would I still be getting some sort of output even if one was bad?