Could be the tubes. Make sure all the other connections are tight. If the amp has auto biasing or some other user method you might change tubes--side to side (left with the right)--Driver tubes as well;see if the problem stays on the left. If it stays on the left--change side to side--the tubes in the pre amp.
Lower gain in left channel.
I just noticed last night that the volume from my left speaker is lower than my right speaker. It is enough that images are pulled towards the right speaker, but it is not glaring. Could this be a sympton of a power tube going bad? I'm running An Air Tight Preamp (with 6 month old new tubes) and a c-j Premiere 11A. I'm not sure the age of any of the tubes of the c-j, I bought it used and have had it for about 1 1/2 years. Would a tube or tubes going bad manifest in this way. There are no strange noises or anything coming from the left channel, just the slightly lowered gain. Thanks for your help.