Sound diffusion panels--do they reduce brightness?

If your room is bright, changing amps and/or cables is not a solution. Although you have bass traps and an open space behind the listening position (all help), your room still has a huge amount of reflective hard surfaces, especially at ear level and above. Adding wide-band absorbers at the first reflection points is a great first step. If that step is subjectively effective, consider more.

Kal

Davehrab wrote: By going into the other room and listening you remove yourself from the extended/sustained ringing in your main listening room that is being caused by the long RT60 times, Echo Slap, and Comb filtering...

Not true. You are still hearing all that but with a very different perspective that affects the timing of those elements and, in fact, superimposes the filtering of the transition to the other room as well as the acoustics of the other room. May be informative but it is not unbiased.

Kal