Diffusion versus absorption behind speakers?

ASC tube traps offer both. The tubes have a partial reflective surface identified by a silver dot. You can change from full absorption to a combination absorption/reflection by rotating the orientation of the trap. I use 2 11x36 and 1 9x72 on the speaker wall. I use the two nearest the speakers with the reflective area towards the listening position and the traps at the back of the room in the corners in a more absorptive orientation. The changes wrought by orientation are readily discernable. My answer then would be both absorption and diffusion. On the down side, they are very pricey.

TubeTraps and ASC offer a free evulation of your room if you send them pictures and a diagram. I've use both as a starting point and it worked well, but as others have stated, it is a trial and error process.

I asked the same question for my soundlab m2's and was advised by rives audio to use diffusion over absorption;I started playing and found both seem to work very well and even hanging rug art behind the panels seemed effective;not really sure what the correct answer really is,but your ear will be the deciding factor.

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.

Depends on room, radation of loudspeaker, no right or wrong just whats right for your room system etc.