Thanks for the detailed write up and test results on this!

I am getting ready to do a very similar experiment in my control room. I already have a ton of broadband absorption (too much in fact) multiple 6" bass traps, a few diffusers in a 12.5' x 9.5' room.

I have run baselines with REW with the default treatment in place (corner bass traps, broadband on the walls, etc) but plan to strip the room and start from scratch - taking measurements with each change.

I am going to sacrifice some real estate with some deep 18"+ pink fluffy traps, but will be looking at Helmholtz and/or tuned mass absorbers for narrow band and deep bass treatment.