Play something mono. Move the left speaker more toward the center or move your listening chair to the left. Also swap inputs to the amp back and forth to see if the preamp has a weak channel. If not swap left and right speaker cables to see if the amp has a weaker channel. Balance controls usually add distortion to the signal. So avoid if possible.
Center imaging problems
I am having problems getting an exact center image. It always seems that the vocals are slightly to the right instead of exact center in most of the music I play, which is very annoying. Does anyone have any good test tracks with the vocals at EXACT center? Also what is the best way to compensate for not getting an exact center image? Balance control? Move left speaker forward just a little?