I have the same balance issues with my setup, I use MK5s and Whitelabels. In my opinion this is a calibration issue that involves mainly the stylii, their connection to the tonearm, and the vinyl you are playing. It's inevitable that out of the factory all of these factors likely will not 'line up', and all I do to compensate is adjust the balance for each deck on my mixer. I find that it doesn't vary that much between the different vinyls I play, so I can just leave the balance settings on my mixer at a certain position to keep each of the stereo channels balanced. No biggie really

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