Read the article " Room with no boundaries" and if that peak your interests, Master Handbook of Room Acoustics.
Early reflection from nearby boundaries arrive early in time and high in intensity. Those are the clues that we are in a small space.
Use diffusors to spread out the 1st reflection over time and reduce the peak intensity. By having some component of the 1st reflection arriving later in time and lower in intensity, your ears are fooled into thinking that you are in a bigger space, where the reflections arrive later and softer in intensity.
Bass traps or absorption would not create a larger stage. It will create a more focused soundstage.
Using non absorptive diffusor to the side or behind the speakers can create sound source broadening but do this with care as soundstage can lose focus.
Use absorptive diffusor at the 1st reflection point of the contralateral speaker, this would cut your crosstalk (eg. reflection of the right speaker off the left wall into your left ear), and help with widening soundstage.
Cables, electronics, AC regeneration or conditioning may help but will not overcome the bottle neck in room acoustics.
Resonators may also help but but that would be a mean a second sound source and can alter spectral balance.
