hey there, this is regarding a guitar friend's carvin combo amp. i do not know the model, but know it is about 6-8 years old, a two channel 2-12 tube combo amp. anyway, recently his amp has not been putting out low end for his guitar, there is a noticable difference in that it is not so full sounding anymore. i looked at the back and there are 6 smaller tubes, and i think 4 bigger tubes. the second small tube does not glow as orange as the others while the first glows somewhere between the second and the rest. there are no noticable differences in the tubes, even the large ones other than that. my questions are: are the small tubes for the preamp? do all 6 control both channels? with this information, does it seem that 1 or 2 tubes need replaced? does one faulty or loose tube (kinda scared to jostle it around too much) affect tone in that it just reduces a certain frequency but otherwise sounds normal? am i completely missing something else (it is not the guitar) like a bad pot or something? any and all input is hugely appreciated.