Hmmm...

Just as a reality check, you swapped the connections between the channels as a test, right? I just want to eliminate an external cause or perception. You may also want to check your MP3 files and their encoding. There are some test tracks floating around out there that are specifically for calibrating balance - it might be a good idea to look at those first. Because, to be honest, there is not very much you can do (that I know of) to adjust the gains on the RCA jacks via hardware...