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.  

Many well recorded albums in classical and jazz are able to give, if the room is well treated and especially mechanically controlled, an imaging and a soundstage outside of the speakers alignment vertical plane, back/front and outside of the speakers to the left and right in many cases...

In some recording the soundstage is oriented more in the front/back direction outside of the speakers vertical plane , in some others recording more in the left /right direction and outside of the speakers vertical plane also.. In many case it is a mix of these two....My soundstage is NEVER between the speakers only, save for mono recording...

My system value is 500 bucks but well chosen... 😁😊The reason why my S.Q. is so good is acoustic mechanical control and passive treatment...Not out of phase phenomenon nor distorted reflections in my case ... 😁😊

In some very good recording the sound even come from my back and behind the speakers and all around me AT THE SAME TIME....This recording for example , you can see the singer walking AROUND you and turning their head singing...:

By the way the more satisfying acoustical factor after naturalness of timbre to enjoy and have it right is not merely imaging and soundstage but LISTENER ENVELOPMENT, the sensation of INTIMACY there is in headphone and the sensation by the listener to be included in the sound source, this sensation is possible if we control the timing between lateral reflections and back and front reflection amount and timing...

Acoustic and control of timing between lateral reflections but also back/front reflections proportion between what is aborbed and diffused is the key...

And advice: dont upgrade to costlier components to reach a bigger soundstage...

The gear is not the main cause of the lack of soundstage and imaging most of the times but defect in acoustic treatment and control...

Never upgrade BEFORE acoustic studies and embedding control and passive treatment done right...You will be astounded by the unknown potential of what you already own is able to reach and do...

No more upgrades for me and no more of my 7 headphones, because my 500 bucks system is enough anyway and beat them all on all count even intimacy....

It is useless to name the brand of my gear and boasting about them, because the main reason of my S.Q. is ACOUSTIC and basic psycho-acoustic knowledge...

But it is mere trivial and common place fact that we cannot enjoy an acoustical factor which is not in the recording to begin with, that goes without saying ... 😁😊

It is less common knowledge that you cannot enjoy an acoustical factor which is in the recording BUT cannot be listen to because of a bad room acoustic treatment and control...People often attribute ERRONEOUSLY this experience, or lack of, to a piece of gear and throw their money in a not so usefull costly upgrade.instead of experimenting with acoustic...

More easy to pay and plug than to think ,Alas!

Ultimately I think it is recording dependent. I've heard tracks extend beyond the speaker boundaries, so my system is capable of it. Most tracks are within the speaker boundaries, however.