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

Nah, just a WAG based on your description of the sound you are getting, i.e. too bright and poor bass, etc. If my use of the word 'gross' offends you (and it must since you seem not to focus on any of my other posts) then I will gladly admit it was probably a superlative and change it to 'minor'. That help?

IF the speakers are properly set up in your room, something that was suggested in many posts in your 2 threads, by myself and others, IF the electronics are high quality and 'matched' properly, then, the only thing left is the speakers which must be deficient for your purposes. If the speakers are not deficient for your purposes then you really have a conundrum. Good luck solving it. 

B&W speakers with the diamond tweeter are very bright to my ears. The solution is to use a warm preamp and amp combo. A number of years ago I heard one of the larger pair at Best Buy Magnolia with a Rotel preamp and amp that sounded very good. The very reasonably priced Rotel components actually sounded better than the pairing with high priced Mac tube monoblocks. B&W and Rotel are closely aligned companies, and B&W has been known to voice their speakers with Rotel components. If there is a store near you I would call them and see if they have the pairing for a demo. Costs you nothing to hear for yourself.

B&W speakers with the diamond tweeter are very bright to my ears. The solution is to use a warm preamp and amp combo. A number of years ago I heard one of the larger pair at Best Buy Magnolia with a Rotel preamp and amp that sounded very good. The very reasonably priced Rotel components actually sounded better than the pairing with high priced Mac tube monoblocks. B&W and Rotel are closely aligned companies, and B&W has been known to voice their speakers with Rotel components. If there is a store near you I would call them and see if they have the pairing for a demo. Costs you nothing to hear for yourself.

Stay away from noisy graphic EQs and get a Schiit Loki. Four bands, under $200.

Steve Guttenberg has one and loves his.

B&W upgrade kits….

"These speakers and all the tweaks offered by other forum members (and now upgrade kits) suggests there are just some flaws with the design. Is this tweaking and upgrade path normal with speakers?"
yes tweaking is VERY normal now for many speaker brands and models no the comparison to some turntable w/ issues is irrelevant . there are experts who have done the research and come up w/ fixes when  you have a speaker you like it makes sense to resolve some issues it has and keep it . sometimes it is cheaper than buying a better model sometimes not . but at least you know it is the best it can be lets say you buy a upgrade model it may have issues too!