When I swap the left and right channel tubes, the noise goes to the left channel.
@earthtones This statement suggests that the tubes you installed in the channel in question were in fact defective.
I've never seen a fault that could cause a tube to go crackly like this. So IMO/IME the simpler explanation is you've had bad luck putting the bad tubes in the same channel.
Swap a known good tube from the good channel and see if that goes bad. Then you'll have your answer.