I think you are right about that being a common situation - asymmetry, that is - and I haven't experimented enough to be able to provide an answer. I have never bothered to try to duplicate the acoustic characteristics of a wall.
I do wonder if you have considered a center line diagonally out from a corner, though. I have seen quite a few situations where that was the only option enabling lateral symmetry and the resulting sound is often very good. I also believe that in this case the center corner behind and between the speakers really does benefit from substantial wideband absorption.