I'm not saying it *is* azimuth in your case because there are so many variables, but azimuth isn't a bad place to start.
I had a similar problem, and when I got a headshell with azimuth alignment (for a Technics DD TT) and adjusted it, the channel balance and imaging snapped into place.
I had a similar problem, and when I got a headshell with azimuth alignment (for a Technics DD TT) and adjusted it, the channel balance and imaging snapped into place.