Switch the right and left cables and see if the problem is in the opposite channel. You can narrow down the problem by taking the cable that seems to be higher volume and using that to connect just the left channel or just the right channel to figure out whether it is the preamp, power amp, or cable that has the problem.
One Channel Lower Output With XLR Cables
I switched to balanced XLR cables from the RCA unbalanced types. When I did this, I noticed via the amp meters, that the right channel had a lower output then the left in all situations, CD-player and tuner. When I switched back to the RCA cables, both channels went back to the same exact output levels. Is this caused by the cables or the balanced outputs of the pre-amp or amp? Does a one meter length of cable really make that much difference in using balanced or non-balanced cables anyway? Thanks.