My apologies for stating the obvious, but given equal-level signal source to both channels, balance set in middle, you'll have equal level of output to your speakers, IF AMP IS FUNCTIONING PROPERLY. If there's a noticeable difference, then time to find out why. With stereo programming, depending on source material, sometimes one channel is going to be louder than the other. Make sure this isn't what you're experiencing before taking further action.
I seem to notice this mainly on the college jazz station I listen to, so this may be the issue. I'll check other sources like CD and LP