I had a volume balance problem, perhaps it is your tubes.
Turned out to be one of my Cayin A88T's 6sl7's (it also has 6sn7's).
had several used GE 6sl7 downstairs, that proved it was just the tube, not the amp or anything else in the chain.
bought new 6sl7's from Brent Jesse.
Picked a Sylvania model because the chrome tops would look different from the 6sn7's (I had mixed them up once).
Sylvania did not sound as good as the unidentifiable Cayin tubes, or the used pair of GE's.
Brent had me break in the Sylvanias for 60 hours (no signal needed, just leave then on). Still not as good.
I picked alternate GE, I paid postage only, Brent gave me full credit, and, the GE's I ordered and love were half the price.