B&W 802D1 - How can I tame the brightness?

Sell them, buy a pair of Spendor Classic 1/2s, and start enjoying your life and music again. Life is too short to deal with gear you don't like! No cable, power confitioner, tube, or amp/preamp change can sufficiently or fundamentally change a speaker that was engineered to be inherently bright... simply will not happen.

OP here's some info on Franco Serblin

https://www.stereohifi.nl/loudspeakers/stereowise-loudspeaker-f/stereowise-l-speaker-franco-serblin

@onehorsepony 

Rockport speakers are end game speakers. I highly recommend you put them on your list to audition. Good luck ! 

Current components consist of:

Mark Levinson 5101 SACD player

Audioquest XLR Water interconnects to preamp

PSaudio BHK Preamp

Audioquest XLR Water interconnects to monoblocks

PSaudio BHK 300 monoblocks

Audioquest Inidigo Indigo bi-wired speaker cable

B&W 802D (1st gen diamond)

I note that you are now also questioning the bass response as well as the mids and highs. I suspect you have 1) speaker/listening position set up issues and, perhaps, 2) a gross mismatch of preamp/amp and speakers. Do some on-line research. These issues are covered extensively.