My room is about the same size as yours, and due to the location of a fireplace there is no choice but to put the speakers on the long wall.
This is a long thread, so maybe I missed a relevant comment. If so, please excuse the repetition.
I wasn't sure whether to use absorption or diffusion behind the listening position. GIK recommended diffusion, but just in case, I ordered three large 4A Alpha panels, with the backs optionally covered with fabric ($12.00 per panel) instead of the default white muslin. That way they could be used for diffusion + absorption or turned around for absorption alone.
After experimenting, I ended up mounting them on the wall with the diffusion side out. That may or may not be the best approach for your room, but at least the reversible panels allow you to easily find out.
This is a long thread, so maybe I missed a relevant comment. If so, please excuse the repetition.
I wasn't sure whether to use absorption or diffusion behind the listening position. GIK recommended diffusion, but just in case, I ordered three large 4A Alpha panels, with the backs optionally covered with fabric ($12.00 per panel) instead of the default white muslin. That way they could be used for diffusion + absorption or turned around for absorption alone.
After experimenting, I ended up mounting them on the wall with the diffusion side out. That may or may not be the best approach for your room, but at least the reversible panels allow you to easily find out.