Why are my B&W 803D speaks feel bright

Almost any speaker can be made to sound good or bad depending on set up.

Because they are. Try toeing them in a lot.

Rotel and B&W used to be imported by the same distributor so they were almost always sold at the same stores. Therefore they usually ended up together.
But if you want to tone them down, without spending any money, toe them in to where the tweeters cross in front of your listening position. You will probably want to spread them further apart if toeing them. Toeing them in will also give you more depth and center fill.
If it doesn't help, you've only spent a little time.