One Sub or Two?

Two.

For two channel high-end systems to get optimum benefits always use two subs. 

Check out NSMT band-pass subwoofers, which load a room differently then ported or sealed subwoofers. 

Definitely two. If you wanna read more on why look up Todd Welti who’s a foremost expert on subwoofers and read his research. For a recommendation, you can save yourself big $$$ by getting two excellent Rhythmik F12G subs for $2500 and get down to 14Hz. Several people here bought them on my recommendation and are all thrilled.  Or you can pay a ton more.  Best of luck.

https://www.rythmikaudio.com/F12G.html

Get two.  I have two RELs in a small space and it’s pretty awesome.  Many here really like Rythmik as well.

2

I think I'd wait and see how they sound in your room.  I wouldn't expect a speaker like an 801 D4 to need much help for two channel. I think you'd need really good subs to gain anything. It's interesting that REL's website doesn't have a recommendation. They recommend their model 31 for the 800 D3. It's a $7500 sub. I'm not saying you need to buy a REL, but you need a really good sub. 

How big is your room? How much do you want to spend? 

Two subs for music. 

I went with F12SE Rythmiks because of the ability to dial them in variably (rather than 0 or 180, as the case is with REL subs) to a pretty tricky room. I got a pair. Love them. I have an older REL, too, and it is my third sub. The subs are coordinated using MiniDSP (link is about subwoofer integration).

Again, REL has a huge fan-boy contingent but not many models have variable phase. REL has 0/180, mostly.

A simple 0/180 switch allows for a complete reversal of the subwoofer’s polarity, effectively inverting the signal. But variable phase control on a subwoofer allows you to fine-tune the bass output to blend seamlessly with your main speakers, maximizing bass response and avoiding cancellations at the crossover frequency.

Two REL's, which ever models you can afford.

I have several systems and they all have two REL subs and I'm very happy with them.

Information about the size of your room would be helpful.  Based on the speakers and the HT / 2 channel mix, assuming a larger room, more is better.  No need to stop at two.  Highly recommend Rythmik servo subs over REL.

This info is all super, very appreciative!

Edit to include:

1. Room is not dedicated listening room, various wife-driven things like couches, coffee tables, lamps, etc. and is approximately 30Wx40Dx12H.

2. I normally use a sub for 2 channel listening as the B&Ws don’t quite cut it to my ears.  They excel in the midrange.  More important to me to have the music sound as correct as possible than the HT listening experience.

Again, many thanks!

1) most definitely two. evens out the response, increases dynamics and avoids localization.

2) I had two REL T9i and they sounded very good but missed the really low end notes and authority I did not know existed until I tried the SVS SB 3000. The SVS SB 3000 is a wonderfully musical sounding subwoofer with that really low end power and authority that makes it a real subwoofer and an exceptional value,

30x40 feet? Start saving for S class RELs.

I have one Rythmic 15 in sealed sub. No space for 2. The speakers are high passes at 50 Hz and the sub is low passed at 50 Hz. 
 

The sub integration is seamless and you really can not local the sub listening.

I use 2 REL subs, had 1 for year then added the other and significant improvement... a single sub can sound quite good if you have a single listening position you can place the sub for...

For a room of that size, consider two (or more) Rythmik F18 or G25HP subs depending on what budget and room (and wife) constraints allow.

What ever happened to distributed sub arrays, swarms, and the more the better?

I agree the 4 sub swarm is ideal, but we are not all able to do that...

It probably depends on the room, but I'm surprised that I'm the only one  on this thread who thinks that variable phase is important. I may be the only one who measured with REW, but I could see that variable phase was critical for integration.

If your budget permits, I'd recommend two subs.  I have a McIntosh tube preamp and McIntosh solid state power amp, and a pair of JL Audio F113V2 subs, and that combination works great...with my Focal speakers.

The best answer is at least 3. On a speaker that goes deep the question isn't so much bass extension but countering room affects. The best place for the B&W speakers is almost certainly not the best place for the subs. Rooms have spots with bass resonances and with bass nulls. You can equalize resonances. There's nothing you can do about nulls except move the bass systems which you can only do with separate subs. In your system the subs are mainly to get rid of the nulls and you do this by moving the subs. The more subs the easier it is to do. Rule of thumb for your quality system is 3 subs to start. You could need 4.

For a 2 channel music system I recommend 2 subs. 2.2 sounds better than 2.1 in my experience. A lot of good manufacturers depending on what size you want and your budget. I have SVS SB1000 Pros which fit my space and budget. Love their app for control and their customer service for support.

Subs are available at widely varying price points and the lower you go in bandwidth or in desired SPL across that bandwidth the more you will spend. If your sub is supporting music reproduction only, there is very little point in having extension below 16 hz as that is the lowest frequency any instrument can produce as a fundamental to my knowledge. If you are talking about sound effects in movies, I am quite sure many movie soundtracks exploit frequencies below that though I have no intimate knowledge of which movies and how far below 16 hz movie sound tracks might go. You might take a look at the new SVS 17" sub while certainly not cheap, the one review I read on them was quite impressive claiming vanishingly low distortion at even sub-sonic frequencies. They also have a try it in your home for 45 days I believe. Further there is a control app for their subs that allows tweaking its performance from your listening position that is quite sophisticated. Between the app and their home trial, I'd give them a vote for being a low risk purchase. I have a pair of their PC2000 subs in my office/music room and love them. Not cheap, but quality usually isnt.

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.