Why are my B&W 803D speaks feel bright

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.

move them around.

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.

I agree with those above recommending you try a different preamp.
I have had better results with Rotel power amps using preamps from other manufacturers. I would be surprised if your Meridian was the cause of brightness or edge.
I have not heard the803Ds, but you can definately get a more refined treble with another preamp.