I also had a similar cable problem. It turned out it was one of the hot leads shorted to ground. All I had to do was remove the connector, reposition the insullation and it was as good as new--didn't even have to get the soldering iron out. Do as Panzercat suggests--and if it's the cable you can start with a simple inspection of the leads at the connector. Hopefully, it's a simple grounding 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.