Balance as in left/right, or balance as in front/back?

Have you set them to Full Mode, it is  more balanced than the Ambient Mode.

Your question could be clearer is you adhere to the Sonos lingo. You own a soundbar (that is more than a center speaker because is also does front left and front right), a Sub (or Sub Mini) and indeed rear speakers. Full mode as mentioned above will give you stereo from the soundbar and the rears. Is that what you are looking for?