I would swap the left and right RCA coming into the amp first. If the sound is still louder on the right than it's not your source. Checking bias is always a good thing and if you haven't done it I would.
If that doesn't do it try signal tubes first as you don't have to wait as long for them to cool down. If that makes no difference you could swap the two left KT88's into the right position and the right tubes to the left position. If you do this you must re-bias the amp.
Good luck
If that doesn't do it try signal tubes first as you don't have to wait as long for them to cool down. If that makes no difference you could swap the two left KT88's into the right position and the right tubes to the left position. If you do this you must re-bias the amp.
Good luck