I just recently experimented with diffusion panels on the side walls. They did not work out at all. I was using the 23" x 23" QRD diffusors (the ones from ATS Acoustics). It took some critical listening, but what happens with the diffusors on the side walls is that they will emphasize/boost some frequencies and then subdue/cancel other frequencies. I believe when the sound waves are bouncing around in the slots, it creates enough of a delay that it will cancel out the waveforms of some of the frequencies by the time it reaches your ears. Turning the diffusor upside-down so that the deeper slots or closer or farther from you will totally affect the sound as well. I could not get the diffusors to work on the side walls properly whatever I tried. Since I had to put them somewhere, I put them on the back wall. They seemed to work okay horizontally, because vertically they still cancelled out some frequencies. I would be very careful with using the diffusors. Always test out locations first and listen CAREFULLY before permanently mounting them.
Absorption is good -- up to a point. I have found that just adding a couple too many panels could throw the balance of the room off so that it is absorbing too much sound. What happens is that it starts to suck out too much upper mids/highs and you lose the excitement of the room. It’s a fine line, but once you’ve crossed it, the room becomes dull.
I found the best placement in my room is to have a 24" x 48" x 2" panel behind each speaker (they are in the front corners). Then a 24" x 48" x 2" panel in each of the back corners of the room. Since the speakers are corner loaded, it prevents standing waves which really make the midrange blare/resonate. I found that doubling up the panels in the back of the room (by stacking them on top of each other making a 96" high panel or by doubling up the thickness so that it makes a 24" x 48" x 4" panel) will push it over the threshold where it’s sucking too much sound out of the room. I have a fully carpeted floor, so that is absorbing a lot as well. These are cheap foam panels. I have some Roxul Rockboard 80 on the way, which behave very similar to the foam but with much better bass frequency absorption.
I don’t know which model you have, but the ESL speakers look big, and they might be reflecting sound waves as well as generating them (I don’t know). I’m not sure about putting absorbing panels behind them, unless you put them up towards the ceiling. Putting absorbing panels behind you sounds like a good idea (since you are so close to the rear wall). Maybe also look at points on the back wall where the speakers are directly aimed at and put absorbing panels there? Or maybe diffusion. Different things to try, lol.
