Diffusion versus absorption behind speakers?

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.

I have tried rugs, curtains, furniture etc as "room treatments" but in general I did not find significant improvements. Professionally made acoustic treatments (ie, GIK, RealTraps, etc) are designed (with measurements done in certified acoustic labs) to cover appropriate frequencies and are very effective. In my experience, furnitures, books, rugs, etc are crap a shoot and are less than satisfactory unless you have glaring acoustic problems (eg, room with extensive glass windows) in which case some minor improvements can be heard. Yes, some professional stuff are very expensive, but there are some (eg, GIK) who sell reasonably priced treatments.

But let's get back to my original question regarding room treatments behind the speakers. I don't mind if you mention specific products as long as you are not selling them. Thanks.

Some say that 60% of what you hear from your system is your ROOM.

I was told to try some wool carpets or blankets behind my speakers to keep my downstairs neighbor happy. I tried some thick 2.5' X 3' cotton rugs behind my speakers and she can't hear a thing now. If it can keep the sound from running down the walls (and I do play it loud at times), then one can imagine how it 'tames' reflective sound.