Improving 2- Channel Soundstage

Dealing with the acoustics is the best solution as @auxinput and others have mentioned.

When you begin mixing 2-channel and HT concepts, it quickly feeds the notion that HT can't accurately playback stereo information...which is true when you don't know what you're doing...and most don't. The vast majority of the HT products don't have "surround modes" equipped to accurately reproduce the nuances of the recording. I'm not persuaded spilling rhoduim everywhere will do the trick either...but to each their own.

@kalali Taming your acoustics will produce noticeable results. I would recommend staying away from the surround modes unless you're willing to invest in the electronics that will do the jobs correctly.

I've found that a center does more harm than good with what is traditionally considered the 'soundstage'.

The autocalibration routines in most AVRs are going to set the center volume to be equal to that of the fronts because from an HT-centric perspective, dialog is equally important as L/R content.

But for music, what ends up happening, from my experience, is that this is too high a level and the soundstage becomes non-descript, almost 'mono' sounding.

For multi-channel music, or some of the HT modes (such as NEO6 or PLII, etc, et al), I manually lower the center level until some semblance of L/R separation and soundstage 'returns', then save the setup in one of the memory settings.

For 2-channel, all the funny sound modes get turned off and I get much better soundstage results with just L/R.

@erikt That seems to be the trade off when dealing with today's HT equipment. I’m a firm believer that only a few companies have gotten their sound modes correct. Everything else is exactly how you described it, funny sounding.

Consulting with an experienced stereo professional (not a Home Theater installer) may provide the quickest outcome you're looking for.

If you have experienced great two channel sound staging at a store or a show one should take note of the environment, equipment, and most importantly the recording. With that recording you now have a sonic baseline tool to begin with since sound staging is very recording dependent. 

I have found that the delicate two channel sound stage always suffers when Home Theater equipment is present between the Stereo mains. I have much greater two channel staging by a majority of reflecting rather than by damping the output. The opposite works better in my dampend 7.1 HT room where multi-channel SACDs are much more enjoyable to listen to compared to the two channel room. 

I always begin a two channel setup by dividing the room into thirds and placing the speakers without toe-in on the front third line and a wheeled office chair at the rear third line. Experimenting with speaker location along the front line and rolling the listening position in and out can give you a basic idea of how YOUR room and locations can affect the presentation.

Room correction equipped Home Theater can be far more forgiving compared to a two channel system. Even so, by keeping the HT speakers at constant hight and distance from the listening position uses much less room correction and will usually provide an amazing suround experience even with a very modest system. Remember is only TV.

Good luck with it.