B&W 800 D4's, Room size, & general Q's

You are going to get a 1000 different responses saying one speaker is better with this amp vs that amp.   Welcome to AG.  Wait til you get the dealers pushing their high margin brands...

Do your due diligence and ignore 90% of the these posters.

I rarely post but loved my older 802s with ML amps and No. 39 CD.  Twenty years later and my ears much less refined, I find streaming through Roon/Qobuz with Purifi class D amps and monitor speakers a better experience.

With that said, these new D4 are very intriguing and may have to take a listen to them......good luck with your search.    Listen...don't rely on a bunch of gasbags telling you what to buy.

I have 803 D3s in a room similar to yours. Getting the 802’s or 801‘s isn’t about whether or not the speakers can play loudly enough. Each speaker just sounds different. So choose the speaker not based on the size of your room but on the sound of the speaker. Your speakers are going to need to be about 4 feet away from the front wall and 3 feet from that side wall. For me that means I have to roll the speakers forward when I’m listening and push them back when I’m not. Looks like you’ll have to do the same thing. I would include a pair of rel subwoofers in your set up. Even though the diamond series plays great bass the subwoofers help increase the soundstage. I love my speakers and Pass Labs amp. But it would make sense for you to audition many otherSpeakers to make sure you get exactly the kind of sound that you like.

Regarding B&W, it has been commented upon that their tweeter sounds very bright.  McIntosh tube products are considered by many to be very warm sounding.  I have heard them together, and those characteristics balanced each other out very nicely.

A brand of speaker not yet mentioned is Sonus faber which are unanimously considered to be Italian goddesses in the looks department.  They are fantastic sounding to me. Personally, looks are important in a speaker. 
These are generally comments.  Have fun!   

I have heard the 802D3 and 803D3 both on McIntosh amps (mc611). 
The 803D3 has no bass… for me me at least. It sounds thin. The 802D is better and with your budget I would be looking at the 800D, your room is fine. 
I have owned B&W in the passed and it really wore on my ears. I was happier when I moved on a long time ago. I really like McIntosh amps, though they are not popular on this site. I A/B a few amps and bought the MC462 which I am happy with. 
Once I had the Wilson Audio Sophia 3, 802D2 and Thiel 3.7 in the same room/ system for a back to back A/B with a few A/B of amps thrown in there too. Moved speakers around to their best spot for each one (awesome dealer!). That day the B&W held their own but it was easy to see they were colored once played back to back like that. B&W is very detailed but the frequency response is a rollercoaster. When you demo listen closely to the upper mids of the 800s and make sure you can live with it long term. 
The thing that stood out in this A/B was the dispersion pattern of each speaker and how it effected sound stage.

Rockport speakers paired with Boulder amps sounded excellent at the dealer. Good luck in your search !