One Sub or Two?

Like most of the others, I say get 2. As for which ones to get, my first choice would definitely be Vandersteen. If they are out of your price range, REL would probably be my 2nd choice.

30X40X12 is a very large room - 14,400 ft3, Two big subwoofers are a necessity to fill a space like that. My only complaints with REL are lack of continuous phase adjustment and their ignoring basic best practices for biamping. To be done properly, there needs to be a line-level crossover that rolls the mains off and rolls the subwoofer in, No speaker-level driven subwoofer can do this. Most HT Amps and preamps accomplish this via a Low Frequency Effects (LFE) output and Bass management - adjustable crossover frequency, level, and phase, often combined with Audyssey or Dirac equalization.. Some sub manufacturers put these electronics in the sub, but this requires 2 sets of interconnects - to the subs and then back to the main power amps. Certainly less than ideal, given that the subs may have to be located some distance from the preamps and power amps. This is problematic electronically, and the interconnect costs may be very significant.

As for me, I am awaiting for a high-end preamp with proper LFE and *shocking* a variable loudness control. Yamaha has demonstrated the utility of that for decades. To their credit, Parasound comes about as close as I have been able to find.

I'm going to go against the popular view here.... and for the record, I have a pair of Sopra 2 and a pair of Rels.

The B&W 801 D4 is a serious, high-end speaker that is tonally accurate, fast and tightly controlled.  It will deliver clear, precise music.  And before people start piling on, yes, it's not perfect and I'm sure you all have your prefs.  Having said that, anyone who says this speaker is flabby or dull is mistaken,  Looking at the specs, your model has 10" woofers, a lightweight mid and the diamond tweeter.  The freq range is 15hz-28khz +/- 3db!  They are not cheap.  List new is over $50K.  So congrats on that.  Next, to the point.

First, the idea of putting a "pretty good sub" with a speaker at this level is ridiculous.  It will not deliver the precision of the 801's.  1, 2 or 10 of subs is not going to matter.  They will smear the currently pristine lower end.  Take a great sound and F' it up.

Second, you have absolutely no need for a sub with these speakers.  Get your system set up and live with it for a while.  A couple of years at least.  You'll learn where your weak links are and I assure you they won't be with the lower end.  If you do eventually want to add something that goes to the basement, buy the very best ONE you can afford.  Then keep an eye out for another to turn up on the market somewhere or save your monies for a second one.  But seriously, you're not going to need a sub and adding anything but the best will degrade a system with these speakers.

I went from two REL 510s to a single Carbon Special and in my system it resulted in an upgrade. Perhaps I never had the 510s perfectly dialed in, but how much can one futz with placement and settings before calling it good? The Carbon Special immediately played deeper and faster, and I couldn’t detect any drop in ambience and soundstage by switching to a mono sub. 

One thing for sure is the 510s were significantly faster and more defined than were my previous Rythmik L12s despite the latter having servo controlled woofers. This probably shouldn’t be surprising though as a single 510 retails for >$2500 while the L12 is now something like $699. I’m definitely a REL convert now. They prioritize 2-Ch music listening when designing their subs and that comes through in their performance. 

The more subs the better, but I agree, with speakers like those, tread lightly - only use very good subs.

I don't have speakers of that caliber.  But I do have 5 RELS in a huge carpeted bonus room (entire footprint of a big house) and it took all those subs to make the room.  

At this point, I truly marvel at the tightness of the bass.  And BTW in terms of dialing them in, the gain on the subs needs to be truly low - can't hear any individual subwoofer.  If you can directionally hear it, not dialed in.