How to tame brightness

The best is to use some absorbers on the wall

It might be that you have an issue with RF pollution getting into your system. I have found that a simple DIY RC filter across the speaker terminals can greatly reduce system brightness and make the sound more musical. Here's how you can try this out:

Solder a .01uF Multicap RTX cap in series to a 10ohm, 1 watt PRP resistor (both available from PartsConnexion for a total cost of ~$4.00). Connect the capacitor end to the (+) terminal of the speaker, and the resistor end to the (-) terminal of the speaker. Take a listen. When I did this I heard more detail and a more relaxed and musical, less bright presentation. I also put them on the amplifer outputs as well after they made a difference on the speaker end - that made less of a difference.

Give it a shot - it's only $4.00 and a few minutes!

I have just replaced the Mundorf Silver/Gold Cap in a monitor Speaker Crossover. An Ampohm Polyester in Oil cap of the same value has smoothed out the brightness of the MB Quart Metal Dome.
I believe your Speakers use a metal dome with a simple First Order Network. A replacement of the one Polypropylene cap in each speaker will tame the brightess and enhance your Middle Range.
I've reached the 100 hour mark and my monitors are much more enjoyable to listen to.

I agree with the post from Duke as well. I have a pair of Aerial 7Bs and was having the same problem with too much brightness. I tilted my speakers back at first by just putting a couple of 1" wood blocks under the front feet of my speakers. This made a substantial difference and really help the tonal balance. I also ended up getting rid of my upsampling dac and purchased a nos tube dac, now this really fixed the issue. Good luck!

I also agree with Duke's suggestion to felt the tweeters. Just go to any fabric store and pick up some felt and double sided scotch tape. You can choose a color to match the speakers.

Carefully cut a hole slightly larger than the tweeter and apply it to the speaker. You can then add additional pieces, layering the felt until you reach the sweet spot in terms of what you want to achieve.

Completely and fully customizable sound in terms of what you are looking for at a cost of around $6 to $7.

I've done it. It works. If it doesn't, you're not out very much.