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

IME, yes. I have used both the rear panels and those at the first reflection points. Speakers are well away from the room boundaries. I have also used the free standing Furutech panels. The latter can be moved around and give you some idea of what improvement can be made at different locations. I use one directly behind each speaker and in the center between the speakers. I use many different speakers and amps and have found the improvement using the panels not matter what the 'chain' is. Getting room treatment right takes time and effort. I've use EQ wizard software to do room analysis and that helped me 'see' what I was hearing.

Two other issues are the equipment and the recordings. You didn't mention the make and model of speakers or the other equipment involved. The reality is the current fashion for a lot of audiophile gear is to sound bright. That's not meant to imply everything out there is bright, but the odds are not unfavorable.

I imagine you intentionally chose the gear you have now so I presume it sounded correct to you at some prior point. You might want to compare that previous environment to the one you're in now.

Recordings are also often bright. Close miking and the generous use of EQ and other effects are very common. Even classical recordings are not immune to this issue. Does your brightness problem occur with all recordings or just some?

The tonal balance of my system is just where I like it on an overall basis, but my biggest frustration these days is the variability in recordings. It never ceases to amaze me how many releases are poor and artificial sounding.

Adding absorptions and diffusion at the first reflection points will help reduce image smearing, better anchor images, increase sound stage, and detail ... but do little to reduce the brightness

The cause of the brightness is ... Long RT60 (decay/reverb) times ... when a note is sustained too long ... it's extended ringing will be irritating and fatiguing and be precieved as brightness

In addition to Long Decay times (RT60s) ... Echo Slap and Comb filtering can also add additonal high frequency ringing to the problem

The easist thing to do is visit Ethan Winer's "Real Trap" site and spend the weekend reading and watching some of the videos on the subject ... this will give you the complete picture

Ethan has made it very simple in plain language any one can understand

Before you do anything try toeing the speakers in so they are firing at a point in front of your head, say 2', this will reduce side wall reflections and give you a good idea if the speakers are positioned well. You may find you need to be closer to the back and/or side walls for low and mid reinforcement/balance. In the pictures of your room the speakers look far from the back wall this might be accentuating the highs. Try turning off the back firing tweeters first and after listening turn down the level of the main tweeters. I'd start with speaker position before spending more money.