If you check that the voltage going into the left and right amplifier channels (or monoblocks) is matched within less than 0.1 volt, there is no point in reversing any of the components upstream. If you also have measured the amp outputs to the speakers and find those to be matched, then its either the room (most likely), the speakers (less likely), or just conceivably a problem with one of the speaker cables.
There are many things you can do to diagnose the source:
Regarding the room, if the room is not already well treated acoustically, try taping heavy sheets over all reflective surfaces, which will deaden the sound but leave you with only the direct reflections from the speakers, and check the center imaging as you do this. It's been a source of amazement to me how much effect reflections coming even from walls in adjacent rooms have on image formation. The ceiling and floor are equally important.
Regarding the speakers, you can measure the amplitude response of each speaker at a group of frequencies across the audio band. I would borrow a measurement setup for this (microphone + measurement equipment) rather than doing it by ear.
Also, you can just use the balance control on your preamp, if you have one, to offset the apparant gain difference. This is cheap EQ.
Best luck.
There are many things you can do to diagnose the source:
Regarding the room, if the room is not already well treated acoustically, try taping heavy sheets over all reflective surfaces, which will deaden the sound but leave you with only the direct reflections from the speakers, and check the center imaging as you do this. It's been a source of amazement to me how much effect reflections coming even from walls in adjacent rooms have on image formation. The ceiling and floor are equally important.
Regarding the speakers, you can measure the amplitude response of each speaker at a group of frequencies across the audio band. I would borrow a measurement setup for this (microphone + measurement equipment) rather than doing it by ear.
Also, you can just use the balance control on your preamp, if you have one, to offset the apparant gain difference. This is cheap EQ.
Best luck.