Decapg - Please DON'T plug TT outputs into preamp OUT! What I was suggesting was a pretty obvious diagnostic that probably already occurred to you...
I assumed you had an outboard phono preamp. What I was suggesting is just reverse the left and right turntable output cables...plug left TT out into the right channel phono preamp INPUT. Plug right TT out into left preamp INPUT. If the lower volume imbalance DOESN'T move to the right channel this would implicate the preamp (or something else downstream). If it does move, it means the problem is with the TT. If you have an outboard preamp, you can try the same exercise. Reversing preamp L & R channel outputs going into amp inputs. Hope this clarifies