How far off center? I experienced this same problem nad couldn't remedy it until I broke down the whole system because I needed to paint the walls. Initially, it seemed like the vocalists were off to the left side by half a head.
When I put everything back in, the layout of furniture near the center audio rack was slightly changed, but the center image had aligned to dead middle. The only other things that may have contributed to the shift was disconnecting and reconnecting all my cables. I had also placed contact enhancer on all the connections. So it could have any number of things that corrected my problem. Room acoustics, a bad cable connection to begin with, contact enhancer or pure luck.
When I put everything back in, the layout of furniture near the center audio rack was slightly changed, but the center image had aligned to dead middle. The only other things that may have contributed to the shift was disconnecting and reconnecting all my cables. I had also placed contact enhancer on all the connections. So it could have any number of things that corrected my problem. Room acoustics, a bad cable connection to begin with, contact enhancer or pure luck.