You are experiencing a problem that is inherent in the 'stereo' design. 'In phase' information is only heard between the speakers. Sound coming from outside the speakers appears only when it is bounced off the walls/ceiling and floor or 'out of phase' information is in the recording. In your set up you have no walls for the sound to bounce off to help create stereo width. That is one of the reasons folks like the short wall more. Depth is highly influenced by speaker placement (5 to 6 ft is optimum as I think you know) but if it ain't in the recording, and it usually isn't, you won't hear great depth anyway.
There are devices made to enhance stereo imaging, mostly based on introducing 'out of phase' information that might help, as there are ways of incorporating rear speakers into a two way system which can enhance the sense of sound stage. If you want to hear the effects of out of phase information (only) reverse the plus/minus connections on one speaker. Then you will hear most every thing out of phase - you will note the sound is very spacious but has no central image/focus.
You have high quality stuff so I suspect you might not want to do this but if your up to a relatively cheap 4 speaker solution investigate the Hafler design. It actually works. But it does have a down side. Doesn't everything? :-)