Sound diffusion panels--do they reduce brightness?

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.

Davehrab's advice is perfect...I treated my room after visiting this site and made the acoustic panels to Winer's specs...really works well...BTW I have on panel ceiling mounted just in front and above the speakers. You also might want to consider tube amplification. Happy Holidays! You have a beautiful looking house.

Beautiful looking room! Are you still using the same APC power conditioner for all your gear? I might suggest running your amp direct to the wall first, then if the brightness persists, try doing without the APC or replacing it.