Pre Amp break-in balance issue

I don't think break-in will correct it. If you're pretty sure the speakers and cables are working fine, my suggestion is to connect the preamp's right output to the power amp's left input, and the preamp's left output to the power amp's right input. If the left channel is still weaker than the right channel, the problem is with the power amplifier. If the right channel is weaker, the problem is with the preamp.

If you are a ROON user, then the following thread may help as a stop gap solution.

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I own a Sanders Magtech amp and I use it with a Magnepan LRS+ speakers. What a combo and I am so lucky I changed my mind about selling the amp. I needed to use the Schitt Yggi+ LIM DAC to get the total satisfaction I have now with this gear.

I bought my amp used from Roger Sanders and he was incredibly helpful with my questions and demo requests. I would contact him first since your amp was used.

I am assuming that with your previous preamp the balance was good?

I am not familiar with that preamp, but I take it that it is SS and NOT tube?  Because a weak tube in a certain socket could cause that..  What I do to verify that the signal is the same at both speakers is I put in a Cd with a test tone designed for that and then I set my multimeter to AC volts and probe + and - on one speaker and then do the same on the other and compare readings.  If I've got a real well matched pair of tubes in the balanced input sockets of my preamp, the readings are almost identical to each other. 

This is not a break in issue, but sounds like a bad volume control.

To be sure, swap cables to amp and see if channel problem moves also.

poor description + unhandled shipping by carrier and/or seller probably equates to poor condition preamp.