Why are my B&W 803D speaks feel bright

Because they are. Try toeing them in a lot.

Have you tried room treatments? I'd also recommend getting rid of the Audioquest cables, replace them with Acoustic Zen.

I have used the 803D for the last 3 years - they are not by nature bright - in fact the aspect I most value is their relaxed fatigue free sound. I also use Audioquest cables, so they not the problem.
So look to your other components. Also check your component mains grounding to ensure that there are no ground loops. Try borrowing a matching good quality CD player and integrated amplifier to simplify the wiring permutations.

Years ago I sold Levinson, Rotel, B&W, Magnepan, Thiel, and other brands back in 87-89. B&W's are not bright by any stretch. Thiels to me back then were. I agree with the previous post, try a different preamp. Rotel is good, but not in the same category as your B&W's. B&W's are also fairly power hungry as I recall.
Back then, when I sold a complete system, I tried to keep all components within 10-20% in cost relative to one another. Think of the 'weak link in a chain school of thought'

Don't know much about the cd player you have. Hope that helps.

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.

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.

Def try a tube preamp, I have B&W Matrix 802's Classe amp and ARC LS-7 Match made in Heaven. Never harsh or edgy. Ditch the rotel.

Mac 402 or 501s work very nicely with the 803ds.

The only way to properly determine the exact cause of your issue is through experimentation. But good insight before you do never hurts.

Most of the B&W speakers I had an opportunity to listen were forward in their presentation. I fortunately have heard the 803D and 802D. They were not in the same system so I can not provide a proper comparison between the two. My impression didn't have any notion of brightness, but its presentation has a forward presence that might not mate well with some components (particularly ones that have been described in a similar way). I have heard people using Classe with excellent results and my best experience with them was paired with Naim. Another setup I heard with the 803D was surprisingly lifeless. I had forgotten what the rig was, mostly because it didn't really leave great impression.

BTW, could you provide a more detailed description of the room and speaker setup. It could greatly affect the bass performance and thus make the balance all wrong. With a speaker such as yours, system synergy and setup are two very big factors. I personally have not heard any of your gear in any system so I can not comment on them.

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

I own B&W802D and listen bazzilion time 803. Absolutely not bright speakers (not mushy with diamond tweeter too!). Very, very accurate - they reproduce exactly what you send to them.

To begin with while meridian is very good amplifier it cannot drive such strongly reactive load as latest B&W are. Unless you change your amplifier to more powerful (yet very musical) there is nothing to talk about. For reference only I use Spectron Musician amplifier

Now, then electrically you are OK, lets look onto sound. I never used DVD as the music source so I cannot comment here

Preamp. May be its personal bias but I always consider Rotel as a cheaply sounding, cheap amplification and not on the level with B&W (just B&W Group owns Rotel, Classe and I believe McIntosh - so these amps are mostly used with B&W). I like and use only tube preamplifiers so my advise is to get one (good only). Again for reference, I use Joule-Electra preamp.

All The Best

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.