Diffusion versus absorption behind speakers?

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

Thanks everyone for your responses. I am not a novice on this subject. I know treating corners of the room and back wall behind the listening seat with bass traps has beneficial effects to tame room modes. Also treating first reflection points help in imaging. But I have a specific question regarding the front wall. Some use absorption that helps with better pinpoint imaging. Diffusion, I heard, makes the sound stage bigger but images get larger also. I probably will end up with a box speaker and a panel speaker. So I'm trying to to see which front wall acoustic treatment will help both types of speakers. I get the feeling absorption behind dipolar planar speakers may not be a good idea.

I am more inclined to go with diffusion regardless of the type of speaker involved. As I stated before and Elizabeth has noted too, use bookcases, furnishings, wall hangings, etc, before looking at acoustic treatments. If one of the walls is a big glass window or door, that should be a priority (use blinds or curtains, etc.). If possible, avoid having a very big and flat reflective surface (like a coffee table) between the listening position and the speaker.

Another "free" fix is to listen in the near field by sitting fairly close to the speaker so that direct sound of the speaker dominates over the sound contributed by the room. My preference on that is to move the speaker further into the room. I almost always prefer the sound of speakers when they sit well away from the wall behind the speaker.

I consider extensive experimentation with placement of the speaker and/or listening chair to be the first, and most important step to getting good sound. Almost all difficult "room" problems can be substantially ameliorated with proper placement of the speaker and/or the listening chair. It is surprising how much even a small movement of the speaker, or change in toe-in or rake angle (how much the speaker is angled backwards) changes the sound. If random trial and error is way too exhausting and frustrating, you can try some of the more systematic approaches. A discussion of these approaches is a BIG subject area (google the "Sumiko" or the "Wilson" method of speaker placement).

If all of the non-acoustic treatment approaches fail, start first with tube traps or corner traps in the corners of one side of the room. Unless you build your own bass traps, these can be somewhat expensive. I am personally not inclined to go beyond bass trapping because the costs can get really high and the results are often inconsistent. Plenty of junk on the walls to diffuse sound usually works as well or better than specialized treatment.

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.