stone wall behind the audio rack and speakers

The question that comes to mind in that photo is are the DyneAudios facing each other just while the Utopias play or do they actually play them firing at each other?
But having the rough texure should help. I have an issue where my TV is between the speakers and cause reflection. I made an acoustic panel that seems to help quite a bit.
You may find that acoustic panels on your exisiting wall may do more for your room acoustics. Although a stone wall looks very cool.

Sandstone. No problem, and I do agree. Not having a dedicated room is a bummer. Especially in the age of the ever growing TV.