Cause of Channel Imbalance

Another possibility could be the interconnect cables themselves. It's happened to me. Turnes out to be an intermitant short in one of the cables. Try switching the L and R cables and see if the problem changes channels

Make sure your speakers have not moved - they should both be EXACTLY the same distance away from your listening chair. I like to attach a string to the center of the chair and move it back and forth between the 2 speakers' tweeters. Both directions should yield the same amount of tightness. Try it! ...John

I too had such problem but having in mind my two kids I spotted that ballance knob arn't in the middle:-)

Are your speakers properly connected in phase? Double check.
Is the area around the speakers reasonably symmetrical?
These are two potential reasons.

Will