Absorbtion or diffusion??

As a rule, dipolar and bipolar speakers will do best with diffusion behind them. Absorbing the back wave defeats the purpose of having the omnipolar radiation pattern. Somtimes absorption in the middle (between the area behind the speakers) can enhance stereo imaging, but diffusion should be the main treatment behind the speakers. RPG Skylines, RPG Flatfusors, ASC Tube traps (diffusion side out), would all work well for your application. Diffusion is usually the best way to go for behind your listening position also - but it depends on how far away from the wall you are.

I have mirge m-1 and I use 16" tube traps behind each speaker on back wall and it handle most of the problems that you can find in your room without having more tube traps for your corners, I do have a nine (9") in each back corner and that helps a lot.

Martin Logan Quest z speakers didn't like my Genesis speakers setting couple feet behind them at all the bass wasn't defined and just didn't sound right moving the Genesis out of the way helped considerably where the Logans was approximately 5 feet from the rear wall not treated any way I don't think absorption is something Logans like they may prefer diffusion didn't really bother to investigate it at the time. Have found many speakers like a room lens directly behind the speakers placed against the rear wall it helps clarify their sound.