Center imaging problems

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.

Two theories come to mind. The first is that one channel is louder than the other. You can test one speaker at a time with an SPL meter, and if it turns out one is louder, then you will have to track down which is the offending component. This can be done by swapping out components one at a time, assuming you have redundant components lying around.

The second theory is room interactions. If your setup is asymmetrical, or if large objects in the room are near one of the speakers, it can cause center imaging problems. I have experienced this in my system. It can usually be solved by addressing the asymmetries. Good luck.

Cable out of phase..( one side )???