rrm, I've been giving some thought to things that you might do to 'improve' your soundstage, but nothing I've come up with can create the outside of your speakers soundstage which you initially indicated that you wanted to obtain. Others have discussed set up of speakers and listening position. I believe someone mentioned the Cardas system which is a good place to start, if you haven't already got a good placement of speakers and listening position determined.
For consideration is an add on devise that has been around for a few years and has gotten good reviews. It is a Fusion FOZ. It is tubed unit which allows you to control the out of out of phase information in your recording. In older recordings (and some new ones as well) which seem to have a too centered image, albeit still stereo, by increasing the strength of the out of phase signal in mix in the recording you can get a much expanded soundstage. Wider/higher/fuller. BUT it will not create sound outside of your speakers on the plane of the speakers, but can easily fill the wall behind you speakers. The good news - it is not expensive and it sort of acts as a tubed buffer (if you want one). The bad news is it is not exceptionally transparent and works best, I think, in a tape loop. Further, by increasing the out of phase signal you do reduce some of the in phase specificity. It has a bass control as well if you need it. The unit is a bit bass rich and you might. Underwoodwally sells them if your interested. They are relatively cheap. Something to think about....or not. :-)