BW 801 D4 or BW 802 D4 + 2 subs

@erik_squires 

do you like these subs?  (not sure about the s510 ou s812)

or recommend other brand?

about 5-6k to spend on subs.

With a fireplace between the speakers and porcelain tile floors  it's good that you're going to think about acoustic treatment. You're gonna need it.

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.