I have been obsessed with finding the right room treatments for my very small ( 8.5 feet by 10 feet by 6.5 feet high) dedicated listenting room for 3 years. Various closed box standmounts were the mainstay of my room for 3 years. Power amps and integated amps came and went.
I was constantly playing with room treatments: tri-corner bass traps in all corners, ceiling corner bass traps, 75% of the ceiing covered in absorption ( OC 703 2 inch thick ), first and second reflection points absorbed with OC 703, diffusion on front wall, diffusion on rear wall, combination diiffusion and absorption in different places. It was crazy making.
The madness finally ended when I made the move to open baffle speakers; Tri-Art 3 Opens Reference. My components are nice but not over the top. Krell K-300i, Ayon S10 MKII, decent speaker cables and interconnects. 2 MJ Acoustics Reference 200 subs.
With open baffles the room becomes the speaker cabinet / enclosure. First and second reflection points can't be treated with pure absorption. I found scatter that 2 foot by 2 foot scatter plates mounted on 2 inch OC 703 and covered in wool blend fabric work best.
Having tri-corner bass traps in the corners behind the speakers was not ideal so I removed them. I left the ones in the back corners in place. I removed most of the ceiling absorption and left 2 panels at the reflection points.
By far the most profound improvement ( and the best $2500.00 CDN I have ever spent on this hobby ) was to have 6 quadratic diffusers custom made locally and hang them on the wall behind the speakers. Full diffusion behind open baffle speakers is of paramount importance to get the most out of your open baffles and your room. Also key ( in my room ) is that the distance from the speakers to the front wall ( 42 inches ) matches the distance from the back wall to my listening position. I have pure absorption on my back wall.
The width and depth of the soundstage, the imaging and separation, the layering. It's quite remarkable. Every single time I have a listening session I find myself shaking my head in disbelief.
I found the following article very useful. I'm sure many of you are already familiar with it already.
https://www.linkwitzlab.com/conclusions.htm#:~:text=Open%20baffle%20speakers%20are%20inefficient,bass%20with%20less%20room%20interaction.