How to tame a bright system?

A DSP would be another option, but that assumes that the speakers (tweeters) do not have a lot of distortions that are creating the effect of brightness.

One can certainly get the frequency response shaped however they want with a DSP.

Or sell the B&W and get a used pair of Vandersteens and call it a day.

Well, any tone control is an option.  :)

I have noticed many years back that I am very sensitive to the high frequency sound, specifically the violin and soprano.  I think the response to the frequency spectrum can be very individually dependent.  B&W speakers are very respectful brand with many ardent followers but I could not handle it.  Later on I noticed I could not handle any speakers of which the tweeters are made of metals or ceramics after I auditioned about close to 15 brands with more on the different lines of the products.  At the end, I chose Dynaudio, which has a soft dome as a tweeter, and have sticked with the brand close to 20 years.  However, the tweeter material is only part of the equation.  Electronics makes difference as well.  After settling with Dynaudio, I still had the issue with high frequency, even though much improved.  The next thing I did was to acquire a tube pre-amp.  With the hypothesis that I could be more sensitive to the high frequency, I started to pay attention to the frequency response of different brands.  I paid close attention to my experience to the highest frequency a pre-amp has before its db drops.  Amazingly, I found those pre-amp with narrower frequency at high end, likely deemed to be an inferior design in common wisdom, works for me the best.  In another word, I need to avoid the pre-amp which has an extended frequency beyond 24KHz.  After about another auditions of 10 different brands, I noticed the positive correlation between the level of my listening fatigue and the highest frequency a pre-amp can reach before it drops in response.  Stereophile publishes the frequency response for pre-amp.  I noticed for a specific brand, the frequency response curves are similar for different lines under that brand.  So in case, the frequency response curve for a specific product is not available, the other products under that brand could be a reference.

Go and get integrated tube like Willenston r88 not expensive and have a good review. Your speaker intend to be bright if you didn't match your gear like amp or integrated. Even if you keep changing or try different cables it will come out bright sound. Good luck and hope you resolve yourr6looking for.

Fix your source first that is where you issue is coming from.