I have the same problem, except my balance control has to go the 3 o'clock to lock in a center image. I had several other people listen to my system and they all preferred balance at dead center. I went to get my hearing checked hoping my right ear needed to be cleaned or something (which you should do too if you haven't already), but my ears checked out fine, which really left me scratching my head. (I also don't notice a balance problem when listening to headphones). This issue has remained through a myriad of equipment changes and in three different rooms. Pulling the right speaker forward helps somewhat, but it doesn't solve the problem and I don't like having that much of an asymmetrical speaker setup.
I noticed when I went to shows and some dealers the image was centered with the balance control set to the middle, and what I realized was that in a properly set up room the problem went away. You seem to have addressed first reflecions, but the rooms that worked for me had some sort of diffusion and/or absorption device(s) behind the system -- usually some sort of half-round tower right in the middle or something like that. I haven't tried this in my room yet but will soon. My guess is my ears are shaped slightly differently in some way, which changes their respective sensitivity to in-room reflections. If you take your hand and just change the shape of one ear slightly it's amazing how much sound changes. Anyway, that's my theory. Hope this helps and best of luck. Please let us know if you find a solution.
I noticed when I went to shows and some dealers the image was centered with the balance control set to the middle, and what I realized was that in a properly set up room the problem went away. You seem to have addressed first reflecions, but the rooms that worked for me had some sort of diffusion and/or absorption device(s) behind the system -- usually some sort of half-round tower right in the middle or something like that. I haven't tried this in my room yet but will soon. My guess is my ears are shaped slightly differently in some way, which changes their respective sensitivity to in-room reflections. If you take your hand and just change the shape of one ear slightly it's amazing how much sound changes. Anyway, that's my theory. Hope this helps and best of luck. Please let us know if you find a solution.