IF you do testing be sure to use a tone generator so the exact same level and tone go to each speaker. That should tell you if there really is a difference in gain between the two channels. Then you need to take the necessary steps to isolate where the gain difference is located, it may be a cable, in the preamp, or as you originally suspected in the power amp. But doing an overall gain measurement at like 1 khz one foot from the speaker with a sound level meter at each speaker will tell you beyond any doubt if there is a level difference.
Tube amp bias adjustment advice
I biased my first set of tubes on my 211 amp recently and am hearing more volume from the left channel. Does that mean I need to turn up the bias on the left channel to center the soundstage? Thank you in advance.