How to tame a bright system?

I give you a example of my room accoustics.

The Grey line is the measured frequency response. Do you really think you could compensate (sometimes in the higher frequencies a 14db difference) that with any cables, dac, speakers etc etc forget it it is an endless quest that you go into. My or other speakers sounded extremely bright by using the white target curve my sound became completely balanced. If you have a room without any room modes thats another situation but in 90% of the rooms that is not the case. Measuring is knowing.

Thank you for the room eq suggestion. I tried a YAMAHA ypao long time ago not for the 606s, but for another setup. What I heard was proper response but at the cost of a lot of details. Thereafter, I dare not try anymore. Could you help to comment a bit more on the room eq while maintaining details as much as possible? Should be something added between the DAC/pre and the AMP?

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.