How to tame brightness

Gmc56-Did you see Duke's(Audiokinesis)response?
I think you should give that a try;I think it will address your concerns quite well as I knew a guy who ran Polk SDA's and he did as Duke mentioned with great success.

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!