Only 4 years later i find this post.
Exactly the same behaviour mine has and i've checked everything i can from the schematics.
Rolled the tubes, bought new ones, even had a full cap job but channel 1 is still quieter.
I'm now thinking that this is normal for them although i have a friend who said his was the opposite with channel 2 being quieter. Or it might be the 1 meg volume pot ?
I assumed that they were supposed to be the same in terms of volume with 12 o'clock being full volume, past that was to get the output tubes to overdrive ?