Sound diffusion panels--do they reduce brightness?

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.

Lovely audio room!

FWIW, your room is very similar to mine, size wise and set up wise, as were your problems for years until I got venturesome. Along the lines of Samhar's suggestions but going further, I relocated my speakers which both increased the size and specificity of the soundstage and minimized the first reflection problem from the side walls.

Try moving your speakers toward the side walls until you have a nearly triangular set up (mine are 9ft apart, chair is 10 ft back). Then toe in your speakers until the axis of the speakers cross well in front of the listening chair. Adjust to taste from there. BTW, I use a test record and a SPL meter to help find the best speaker/chair positions to smooth out bass response if I detect a problem.

Invest a couple of hours in trying it. You might be surprised. BTW, location of the chair and speakers from the walls behind them can be critical to the overall tone in both the bass and the highs, especially the chairs location. And, FWIW, I have found no need to introduce audio oriented diffusers or absorbers to obtain a very credible result.

What Samhar said.