Balance control?

Yep, don't count on recordings always having perfectly balanced center image. I especially hear this with 60's recordings when stereo first became ubiquitous, engineers really liked to play with panning, separation, often hear vocalists slightly off to left. Seems like this was common practice, vocalist slightly off to right very rare.

Mono recordings can also expose an off center balance.  Play lots of mono recordings, you should have equal fill on both sides of the center. Try for the nicer mono recordings, those with wider perspective, crap mono recordings very narrow center image.

Stereo recordings can throw one off here, engineers, producers can do some pretty strange things.

Starting with the recorded vinyl record:

Not all have symmetrical balance.

Room placement, toe in, wall reflections can all influence balance.

At the turntable/ tonearm:

Antiskate, correct orientation of cartridge, Azimuth all will have a large influence. One cannot expect exact channel balance with inexact cartridge set up incorrect antiskate or azimuth.

The cartridge:

Ultra inexpensive cartridges cannot be expected to be as precisely accurate with regard to cartridge body to internal generator alignment. Costs are cut. So too are corners in quality..

@theophile its not an inexpensive cartridge. 

My post was general not specific to one individual member. I hope you solve your problem.

O-scopes are cheap these days, mine was about $225 on Amazon for a two channel. You can visually overlay the channels from a test record through the entire signal chain all the way to the speakers.  Easy to spot an imbalance.