Speaker Placement Issue/Thoughts

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.

I have lived for years with a similar situation. The balance seems always weighted toward the left side. I tried everything you can try (speaker placement, component replacement, etc.). I have even had my hearing checked and found no difference, right to left. I have now--after years of torture--simply adjusted the balance control (or the volume controls on my Audible Illusions preamp) to establish the true apparent balance and have learned to live calmly with the situation.
It's living comfortably with the skewed balance control that is the hard part.

I had a similar experience only to find out I had a cable out of phase. Check all connections. Good Luck

Like Rpfef, I've started to become convinced that my hearing is at fault, as the situation has persisted across multiple listening rooms, full equipment changes, and careful interconnect and speaker cable switches (including just flipping the cables connections at the speaker inputs).

Right now, however, I think it is partially due to my room. As you can see from this little mockup (Listening Room Diagram), I've got a 1/2 dining room 1/2 living room with a fireplace (that we don't use because of the stereo) in the front left corner of the room. In order to be able to walk into the room, the couch has to sit at a weird angle, and because of the room shape, there's no way to get the speakers similar distances from the side walls. I toyed with placing the stereo on the left short wall, but unfortunately then the TV (50") will block the room's windows. The S.O. will never let that happen...

As it's currently set up, I'm about 11 feet from each speaker (they're not quite symmetrical though, so it doesn't form a perfect equilateral) when I sit at the far right end of the couch. The speakers are 9 feet apart and 18 inches from the rear wall, and 10" or 16" from the media cabinet that houses all the electronics and a TV. The left speaker is further away from the cabinet because there's a Thiel SS1 sub in between the two.

To address the room issues and center the image, I have to give the right speaker about 4 more db of volume. Or alternatively, I started playing with JRiver's room correction, and I can leave my pre-amp balance equalized and just tell JRiver that the left speaker is 10' from me, but the right speaker is 12' away.