Sound diffusion panels--do they reduce brightness?

It looks like you are discovering one of the most annoying "secrets" about audio. It can literally take years to discover the ideal placement of speakers. Results are quite unpredictable, and shockingly small movement can have BIG sonic consequences.

When using the "Sumiko Method," sometimes moving the speaker an inch in one direction will significantly alter the bass balance of the speaker. This is both frustrating and positive (gives one hope of finding a really good location).

just keep plugging away.

Upgrade your components.

Bj
I just did

Blueskiespbd - have you an update on your "brightness" problem?

I added a QRD diffuser to the side wall to treat the 1st reflection from the main speaker on the opposite side of the room which reduced a bit of HF glare while improving musical details. So, using a miror I found the spot on the left wall that is the 1st reflective spot of the right main speaker and vice versa for the other side wall, and placed a 2ft wide diffuser there at ear level. Worked well as would a thick absorbing panel. You might try it.

I added 2 RPG Diffusers behind the listening area and 2 absorption panels there also. The room is now tame and sounding most excellent.