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.
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.
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.
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.
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.
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.
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.
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.
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...
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,
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.
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.
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.
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?
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.
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