Why are my B&W 803D speaks feel bright

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.

Your B&W 803s are a POS. But I am hear to help you. I will ship you some Thiel CS2.3s and a $1000 in cash--you in turn send me your 803Ds which are a pos. Problem solved

You can try sending an email to member Dougdeacon whose 803Ds sound fantastic. The issue w the brightness from the diamond tweeter very well could be h.f. distortion from your amp at the very upper end of the frequency range, as the diamond tweeters are very revealing. Of course that could mean new amp and pre :-(. BTW, this is a common problem with very high quality transducers. They ruthlessly reveal problems in the rest of the chain.

I have Meridian 605 amps and don't find them bright. I have had several B&W speakers but not the latest generations. You may be having room problems but a simple thing to try is along the lines that Rwware suggested. Go to an office supply store and get a laser pointer; put a tall narrow object like a lamp just in front of your listening position; then fasten the laser pointer onto the speaker with masking tape of blutak so that it points straight ahead. Use the laser pointer to align the speaker so that it hits the lamp directly. Do the same with the other speaker; now the axis of the tweeters are crossing directly in front of the listening position. This alignment was developed by the BBC in the early days of Stereo and is known as "Hugh Brittany Angling" after its developer. Does NOT work with every speaker but will with most. I just thought I had my Spendor S 100s aligned correctly by eye but kept getting too bright a sound. After I had used the laser [ which I should have done first] the sound was noticeably softer but actually had more treble detail. I got the laser pointer idea from the Wilson set up guys who installed my friends Sasha.