BW 801 D4 or BW 802 D4 + 2 subs

That's a really large space that you need to fill and pressurize.  Couple of thoughts beyond the good thoughts from @erik_squires and @tomcarr 

1-Going with the Main + Sub combo will give you options to move the subs around the room and tune the bass.  If you love the sound of the Mac integrated this is probably the way to go

2-The larger B&Ws will push the limit of the integrated without subs.  You may need more power to keep the bass clean esp. at louder SPLs.

3-If you really like a full bottom end and a properly pressurized room the RELs might get you there or they might not.  You may want to consider the JL Audio Fathom line of subs.  They have 2x the power and more usable output then the RELs.  They are known like the RELs to be very musical and two channel friendly.

4-If the porcelain floor remains mostly uncovered the lively top end of the B&Ws will become even more so.  Will you be happy with that sound? Good luck and cheers.

OP:

I'm partial to Hsu with external DSPs, but I like to roll my own when it comes to subwoofer integration.  Here's a good resource outside of A'gon for subwoofers:

https://data-bass.com

801D4....Also, the McIntosh has plenty of power to drive them. You can always add subs later if you are not happy with the bass output. I had a pair of 802D3s and powered them with a Naim Uniti Star in a large room. The bass was adequate but then moved to a CODA 16. The presentation was much better albeit at a significant cost. I was amazed at what the Naim did though in that large of a room. 

Subs always subs. electronic music can only be played properly with subs…. Not kidding. 
 

Anyway, I listened to the 802D4 on McIntosh Mac611 mono blocks and McIntosh 2700 tube preamp. It was ok, the bass was fine, no major complaints, mids were a touch thin (tipped up a bit). Then I came home and listened immediately the same songs on my Revel 228be, McIntosh MC462, with two JL E112 subs highpassed at 60hz. Honestly my much cheaper system was better, much better. The bass was well beyond the depth, quality and slam the 802d4 without subs could not touch. I also think the Revel are more natural/smooth sounding but they do give up some detail next to the 802D4. I would expect the 801D4 to come up short next to subs too. 

The capacitors in the 801 are a bit better still and sound a bit cleaner 

if you can afford them. My dealer Perrotta consulting  carryB&W and give excellent service as well as pricing plenty of low bass.