try disconnecting the preamp and turn on your amplifier. if you still hear the pop, then it's a problem somewhere in the amp. if not, the preamp may be the culprit.
is the pop you hear a single loud pop or is it several? perhaps one of the tubes in the chain (not necessarily the output tubes) is going bad. i know i've had to deal with "spitty" 6SN7's before, but in that instance it was several small pops and never a loud POP.
once you identify which component is making the popping sound, power down and exchange one tube with it's mirrored mate to figure out which tube it is. it's a definite pain the ass, especially since your looking at 8 on/off cycles for just the amplifier alone. perhaps a tube tester is in order for the future. good luck.
is the pop you hear a single loud pop or is it several? perhaps one of the tubes in the chain (not necessarily the output tubes) is going bad. i know i've had to deal with "spitty" 6SN7's before, but in that instance it was several small pops and never a loud POP.
once you identify which component is making the popping sound, power down and exchange one tube with it's mirrored mate to figure out which tube it is. it's a definite pain the ass, especially since your looking at 8 on/off cycles for just the amplifier alone. perhaps a tube tester is in order for the future. good luck.