Diffusion versus absorption behind speakers?

I use absorption but I would think either could work if used correctly. It would depend on room characteristics and your own ears. Apogee told me to leave the wall behind the speakers bare, but I have heard planars that could have benefitted from room treatment.

I had a full "Live end - dead end " set up at one time using Sonnex. I liked it at the time. My present room is 35'x13'x7' and had serious problems. I would also start with bass traps in the corner and go from there. I eventually added 7 more, mostly on the back wall as well as some wool area rugs. the room is still quite live but I can now hear things I never heard before. The amount of detail lost to cancelation due to reflections will astound you. I got mine from Ready Acoustics, who were very helpful, they also will send you a suggested set up if you provide them a picture. They are reasonable and my wife likes the look of their products. No connection, just satisfied customer.

I didn't really find it expensive and I am far from a "cost no object" audiophile. Your room is THE MOST IMPORTANT COMPONENT in your system, if it is bad nothing else matters. I sit in the same place now, which is close to the speakers, that I sat before I treated the room but the sound is very different. Improved? In every respect. If you haven't dealt with room reflections you have no idea of the way they blur the sound. I didn't. If you can try out some good headphones on your system to see what it sounds like without your room interfering. No, headphones and speakers do not sound the same, but you will still get some idea of whatever problem exists. I spent around $800 on my room, could have been less if I had used their DIY option, and there is nothing else I could have done for 4 or 5 times the money that would have given that improvement. My listening friends share the same opinion. Try the furniture by all means, as well as rugs. The wall to wall I inherited when I bought the house turned out to be useless in light of the improvement of the Costco wool area rugs that replaced them. I also had the walls lined with 2+ tons of books and my 3000 LPs and that did not cure the reflections.