How to tame a bright system?

Sure. First off all it is not only software but also hardware that you can add like an carpet, absorption pannels, bass traps. But for practical reasons it is for lots of us not practical. The current room correction software is these days quite good like Dirac, Lyngdorf, REW, Minidsp an mathaudio for instance. Mathaudio is the only free software in combination with Foobar2000 where you don’t need lots of knowledge. For practical use an to find out if Room EQ works for you (an to avoid spending lost of money) connect your DAC to your laptop an connect the DAC to your Amp.

Download the Foobar Mathaudio Room EQ add inn an use Foobar2000 on a windows or OS apple system.

Buy a measuring Mic for instance an UMIK-1 Mic with a Mic standard for best an stable results. Do a multiple measurment as describe on the Mathaudio.com site Vertical.

Listen to the result as you can see in my picture use Room EQ or Bypass.

Decide from there what to do. Most important thing is that you Know what sort of frequencies are amplified or not. When buying a measuring mic atleast you know from than on which speakers to choose from comparing with your room acoustics.

Good luck.

I agree on evaluating your speakers.

B&W has a great reputation and the diamond tweeters irritated my ears.  I think you may find an eager buyer to change the outcome versus 'changing' the signal from the source to the speakers.

Room EQ is a horrible answer. Always ruins detail as you have noticed. Another unavoidable problem with EQ it is impossible to alter frequency response without also altering timing and timbre. It is just an awful "solution" but it hangs around because it does make for nice charts, and some audiophiles are way more in love with their charts than their music.

Everyone knows your problem is your speakers. But you insist on not doing the one thing that will make the most difference. Instead you are determined to focus on cables. Even though you know you already have an excellent cable that makes everything better. Synergistic Research will get you the excellent bass and dynamics you like. There are a lot of used ones out there at great prices, and Ted has been making them so consistent for so long you can rely on that consistency and shop by price. Don't know if that will solve your speaker problem but kind of painted into a corner here on options.

This was as close to Unanimous advice as I have ever seen on Audiogon, a clear majority identified the culprit right away. Yet OP defends the sound of his overly bright B&W speakers, which he “tested”?  Then at home he heard what those speakers really sound like, but rejects the evidence of his own ears.    and the op’s system resource allocation is absurd, as several people pointed out.

It’s an old story.  Pearls before swine. 
 

Sorry OP - this seems very, very unbalanced: a high dollar, highly resolving upstream, paired with $900 metal dome tweeter 2-way bookshelf speakers. The Fire XLR alone lists at $3,500 / 1m. I think a lot of folks would’ve predicted the issues you’re having.

Obviously, you’ve got a lot advice to fix that imbalance. What you have, would be a great upstream for a lot of nice speakers.

On to cables - as others have also mentioned, the Fire is not exactly a relaxed sounding cable. It’s a cable I love, but it’s a little bit more aggressive than even other silver cables in the AQ line. I find my 1st gen Wild Blue Yonders are a little warmer and more relaxed than Fire. The Fire’s predecessor Sky (also 1st gen in my case) was also certainly warmer. The Wind is not the right move - it’s overall similar in tonality to Fire, but lacks the latter’s bass impact and body. And the WEL Signature sounds very much like an extension of the Fire. In fact the Fire sounds like like a WEL Junior. I’ve had both RCA and XLR pairs of all these cables. I’m not sure if later versions of Wild and Sky carry forward the 1st versions’ warmth, but those and the very limited "Horizon" model are worth looking at when they come up used. Quite honestly the v1 Wilds are probably my favorite. The copper cables (I had Colorado RCA and XLR) are a LOT warmer, at the sacrifice of some resolution.