I just went through the exercise of experimenting with tweaking my home theater with diffusion. I had solely absorption panels prior. I have 5 absorption panels spaced out on each side wall and 2 on the middle of the back wall. Each absorption panel is 60 x 48. I decided to get some simple 3D diffusers made from PVC on Amazon just for something to try out with very little investment. i used tape to try them in the various locations, in various combinations. I ended up using them in 2 specific locations, and the improvement is quite startling. My basement home theater room is 20’x12’x8’.
The first position is on the first refection point for the front speakers on the side walls. I mounted them on top of the diffusers in that exist there. This position provided a soundstage that is now well beyond my walls. I had always felt the sound somewhat “tunneled”. After the diffusers, I had to tow in the front speakers in a bit more to what I now consider traditional, yet the soundstage is still way wider.
The second position is on the front wall directly behind the front speakers. This cleared up the sound from the fronts and center quite a bit. However, what was a bit more surprising is how it cleaned up the overall bass within the room, something I would expect from a bass trap not a diffuser.
Any other position I tried them in resulted with a negative impact to clarity or bass response, including diffusion on the middle of the back wall.
“Experts” are all over the place on diffusion vs. absorption. What worked for me is primarily absorption, but diffusion in spot locations. I will most likely replace the Amazon ones with some type of quality curved 2D diffuser, but for now I’m sticking with these. What really became evident was just how important proper treatment of the room is.
If you didn't test the room for its response before every stage of treatment, it can only be improved for specific problems by coincidence.
In looking at your photo, it would appear that the first reflections are reaching the listening positions at the ends of the sofa- this can be verified pretty easily, if your absorptive panels can be removed easily- remove the burgundy ones nearest the back wall and place them ahead of the tan ones- it should make a big difference.
The 'tunneled' sound is usually caused by flutter echo- this is due to short distance between parallel walls and a good way to limit this is by treating first reflections, so the sound energy can't reach the walls where the second reflections would occur.
Bass response can suffer from 'suckout' when the speaker placement and corners cause phase cancellations. Bass traps, meaning thick, densely packed corners or columns, aren't always needed- I treated my room by installing 3" to 4" thick 2' x 4' panels in three corners- the fourth corner is a doorway, so it doesn't cause problems. I used RoomEQ Wizard and pink noise to see the trough in the response and watched as it was reduced by placing the panels. I don't have a problem with flutter, but I did before the panels.
Listening position makes a big difference, too.
Look into 'Live end/dead end' room treatment- if you search online for that term, you'll find a lot about what to do and how to do it.
