Cause of Channel Imbalance

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