Pre Amp break-in balance issue

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. 

My plinius sa100 xlr right channel is weaker after I over drive my Martin Logan request many yrs ago.Thinking maybe the fuse inside are the issue. Replaced the fuse still the same. But on rca no problem.I am not sure what’s the problem.

I am sure it’s not a break in issue on your amp.