Yesterday I finally managed to get some panels behind my listening postion. (see photo) Behind the main seat it's mainly open behind but on the one side there's a stairwell reflecting sound back towards you.

These are the 6" Alpha series with 2D diffusion. What a difference! I was skeptical it would work so well. I knew it would help but was expecting a more subtle change.
I've observed:
-The system is less ’shouty’
-It’s a lot like a bit of pressure has been relieved in the room. I can let volume knob stroll a little higher and the gear seems to perform better when it can stretch it’s legs a bit more
-Center has moved to center - before it was a bit to the left
-Soundstage deepened and moved back just a smidge
Morale of the story: don't forget about your rear!!!
EDIT:this view coming up the stairs is pretty cool!
