What do you do for playback? near field, far field? Speakers angled inward, or straight forward?
IMO, This may be the best way to approach the issue- ie: adapting the
way you play the recordings back over speakers, rather than trying to manipulate the recording to sound best with traditional stereo speaker playback. One approach is what M0k3 mentions above- using a matrix surround decoder such as Dolby PLII, DTS Neo6, Logic-&, Circle-Surround, Hafler-matrix, etc. for multi-channel playback and playing around with the settings.
Another is simulating headphone listening over two speakers using cross-cancellation techniques. These are techniques for speaker playback which eliminate the cross-talk between each speaker and the ear on the 'wrong' or opposite side of the head. There are electronic means of doing this over standard 60 degree stereo speaker triangle such as the old Carver Sonic Hologram system and Lexicon's cross-talk cancellation program among others like in some Meridian and Tact gear, as well as speakers specifically built to do this like a model Polk made some years ago. Those methods work, but can be quirky and require very specific placement of speakers and a listener glued to the sweet spot.
There are other, typically newer cancellation methods that work better partially because they move the speakers very close together so that they are only 10-20 degrees apart, and partially because they use improved cross-cancellation algorithms. Most of these are referred to as stereo dipole systems. Some are plug-in based and can be easily used for computer playback if you move your speakers close together. I haven't used it, but here is a free VST version that can be used via Foobar/Winamp with the appropriate VST wrapper-
http://www.weldroid.net/2010/02/ambiophonics-processor-vst-ambioone.html There are a good number of other available plug-ins available using various cancelation algorithms. Some are better than others. Something like that is pretty simple to setup and may possibly be a practical solution for you.
Some stereo dipoles are built into a single speaker enclosure that includes the cross talk cancellation circuitry like the Emes owl-
Arguably the best cross talk cancelation (and a simple, cheap method, if somewhat funny looking) is accomplished by using a physical baffle sandwiched between near-spaced point-source speakers which runs right to the nose of the listener. You can do this with a sheet of plywood or heavy cardboard. I recommend that anyone interested in how stereo works give this a try, the results are enlightening. It 'turns-stereo inside out' by placing the physical speakers directly in front of you, and produces phantom images extending out to the sides, much wider than the traditional stereo triangle. Just don't let anyone catch you doing it or you'll never hear the end of the ridicule. Here's a sketch I made a few years back to explain it in
another post here at TS-The low-frequency speakers on the sides are optional and just improve the imersion, it's the two small speakers and the barrier between them that are important. If at all interested, I suggest whipping out the plywood to see how it works, then give some of the plugins a try to see how close they can come to emulating the physical barrier.
Information on cross-talk cancellation:
http://www.isvr.soton.ac.uk/fdag/vap/html/xtalk.htmlTry searches with the following terms:
stereo cross-talk cancellationstereo dipoleambiphonicsThe
Ambiophonics technique goes so far as to adapt binaural/HRTF recording techniques with additional baffling to exclude everything but front arrival, then add a second head or additional mics behind the baffle, or room convolution for the room sound.
Have fun exploring the rabbit hole.