If you've already tried swapping left/ right inputs to eliminate a mismatch at the source/cabling, try swapping the leads from the amp to speaker- in other words- remove the cable from the left speaker terminals and move them to the right speaker and vice-versa, leaving the connections at the amp end the same.
If your speakers are toed-in at all, try changing the toe-in angle on one speaker only, leaving the distance from the inside-rear corner of the speaker to the back wall the same-in other words:pivot on the inside front spike. See if the image moves. Don't be afraid to try a drastic change to make it obvious.If there's an equipment rack between the speakers check the distance from it's edges to the speaker- this was messing up the center image on my system at one point- making the distance equal took care of it. I put blue painters masking tape on the floor before I make these adjustments so I can always get back to where I was. If all else fails, move the listening chair a bit.
If your speakers are toed-in at all, try changing the toe-in angle on one speaker only, leaving the distance from the inside-rear corner of the speaker to the back wall the same-in other words:pivot on the inside front spike. See if the image moves. Don't be afraid to try a drastic change to make it obvious.If there's an equipment rack between the speakers check the distance from it's edges to the speaker- this was messing up the center image on my system at one point- making the distance equal took care of it. I put blue painters masking tape on the floor before I make these adjustments so I can always get back to where I was. If all else fails, move the listening chair a bit.