Why are my B&W 803D speaks feel bright

Actually I recently heard a pair of previous generation [ 5 years old or so] 803s driven by a Rotel amp and pre and they were far from bright. Possible the new tweeter would make a big difference but these were in a very reflective room. Rotel makes some quite good equipment at the top of its line, I would rather have their top equipment than Mac SS or any Class D amp. My Meridian amps drove my Apogee Duetta Signatures with no problems; they have trouble with my GamuT L5s, which go down to 2.8 ohms. Driving them hard for long periods will cause them to overheat but the sound is very good. The Revelator tweeter in the L5s supposedly go out to 60 Khz [ I cannot confirm this personally and the bats of my acquaintance have refused to comment] so they do not lack high end. You may just not like the new hard dome tweeters, I do not care for the ones I have heard myself. It is a matter of taste, they are technically very good. In the distant past I used Ionic tweeters, which achieved a similar degree of technical perfection but sounded quite different. As a matter of fact, B&Ws first speaker, the P2H, used the Ionic tweeter, so they have come full circle. I had a pair myself in the late 60s.

You would rather have a rotel ss over a set of Mac501s or even the 402 on the 803ds? Having owned and heard both on this speaker there is no comparison...at least to my ears.

The 803Ds can sound very very good indeed when driven by very good tube amps. I would recommend the best amp and pre-amp you can afford.