Absorb or Diffuse Behind Listening Position?

I think a combination of absorption and fine-grained scattering would probably give best results. One option is the ASC Matrix Panel (as mentioned ty @asctim above). Based on the quality of their other products (I have plenty), I’m sure it’s a great product.

Another option is some 4" thick BAD panels from RPG. I used those in a similar situation and was quite satisfied. There undoubtedly are other similar products on the market.

The objective is that the wall behind you disappears acoustically, neither giving a strong echo (reflection) nor a dead zone, as a pure absorber would create.

Having the listening position against the back wall is not ideal. I assume the OP has no other possible listening position available or perhaps it's a multi-purpose room and there must be a compromise. With that listening position you definitely don't want any reflective surfaces behind you. Diffusers do reflect sound but in a scattered pattern. It's easy to understand why sitting close to a real diffuser isn't ideal. Speak into one that is about 12 inches away and you will know.

What we have recommended before is to use broadband absorbers about 4 to 5 feet wide behind the listening position. Flanking the absorbers can be left reflective but diffusion is better.

Hey folks. Of course I’m working with compromises here. To start with, the compromise is the room: only a "semi-dedicated" listening room; lots of glass; a good bit longer than it is wide; a pair of glass french doors flanking the wall behind my listening position. The system is on the long wall. I’ve talked this through with the speaker designer and we decided that long-wall placement is best, but didn’t completely suss out the rest of the room treatment. Next, the speaker manufacturer’s geometry of 1 (speaker center to center) x 1.18 (perpendicular speaker plane to listening position). If I set up the system on the short wall, paying attention to how near the speakers are to the corners, I might only be 6’ deep in an 18’ long room with a sofa, rather than a listening chair as the center of attention. These speakers are supposed to placed as near to the wall as possible to take advantage of bass boundary-loading, and I’ve put a 2’x4’x2" mineral rock filled panel behind each speaker-pair (I have two pair of monitors spaced and inverted on each other, as recommended by the designer). So the wall behind me - in this semi-dedicated listening room - is the main issue. I’m not far enough for diffusion and more absorption panels might be too dead. I’m setting up some adjustable curtains to take some of the heat off the back wall. Surprisingly, bass problems aren’t that awful. Not sure why. It’s the high-mid and over-bright high frequencies which I’m working to damp at this point. One side wall is good, another is a bit reflective and will probably need something. Ceiling will come last if still needed. Thanks for all the input!

What is your RT-60 ?

6.5 seconds.  But I was faster in college.