Assuming this was never ever an issue prior to getting this new piece, and with it you have a noticeable disparity in the balance of power to each side, I got to throw in with Musicophile.
Send it back. Bummer... drag city... BUT I've yet to exp run in periods wherein the balance between loudspeakers was off and playing it more fixed things. Run in fixes only tonality, extension, and refinement... not balance.
One thing too... turn it on and once warmed up while at the LP... Stand up. Sound different? No? Then turn around & face the other way. sound different now? No? Then it ain't your hearing. Don't laugh. Our ears do change.
Openings to the sides of a room too can cause suck outs and balance issues.
My point being... if it's never been an issue with other gear... it remains it is now... get another, or have it fixed, but don't accept that problem. Might just be the tube but it needs be repaired.