Limited soundstage between speakers

In-phase signals will appear between your two speakers. Out of phase signals will appear outside the speakers. Speaker placement can cause 1st reflection points to create bounce off walls etc and this can/will create a sense of a wider sound stage, however it comes with some expense, i.e. resolution, and tonal changes. Some folks like this, some folks aren't aware of the difference (or care).

In a well set up system you should not hear sound from beyond the outside of the speakers unless it is in the recording. 

Check to see if your preamp or other component inverts polarity. If so you would need to swap the pos and neg leads on your speakers.

How close are your speakers to the front wall? Moving them forward can improve imaging. And using absorbion and diffusion in key areas will improve image, soundstage, and SQ.

My system presents a focused 3D image between the speakers and the soundstage extends beyond the speakers. This can be achieved with proper setup and of course, component selection.

Treat the sides of the room.