Why are my B&W 803D speaks feel bright

Have you tried room treatments? I'd also recommend getting rid of the Audioquest cables, replace them with Acoustic Zen.

I have used the 803D for the last 3 years - they are not by nature bright - in fact the aspect I most value is their relaxed fatigue free sound. I also use Audioquest cables, so they not the problem.
So look to your other components. Also check your component mains grounding to ensure that there are no ground loops. Try borrowing a matching good quality CD player and integrated amplifier to simplify the wiring permutations.

Years ago I sold Levinson, Rotel, B&W, Magnepan, Thiel, and other brands back in 87-89. B&W's are not bright by any stretch. Thiels to me back then were. I agree with the previous post, try a different preamp. Rotel is good, but not in the same category as your B&W's. B&W's are also fairly power hungry as I recall.
Back then, when I sold a complete system, I tried to keep all components within 10-20% in cost relative to one another. Think of the 'weak link in a chain school of thought'

Don't know much about the cd player you have. Hope that helps.

AQ cables are not helping you. For starters, get sth on the warmer side - Cardas Golder Reference cables, van den Hul The Revelation/The Revolution or similar.

B&W designs them this way.