Subs?

+1 on two subs, so size might be predicated on space available and cost.  That being said, ideally I would take two 12" subs over two 10" subs.  I am a REL fan, based on experience, sound  and reliability.  There are many reports here of JL reliability issues.  I have heard the B/W subs and would choose REL over them.  I also replace two Paradigm subs with REL and again they sounded much better.  I am sure a swarm sounds great but even for my excessiveness not practical.  IMHO, get the best REL subs you can afford.  You may hear that REL subs do not have some of the sophistication in adjustment (variable phase and remote control), but I have absolutely no issues with dialing them in.  They add air pressure, bass presence and depth, and augment the soundstage if placed and adjusted properly.

Matching brands is not the leading concern either.

Definitely at least two subs, and three or four is even better if you can swing it.  I’d go for 15” subs but no smaller than 12” because they’ll go lower and work more effortlessly with lower distortion than smaller subs.  I think Rhytmik subs offer the best performance/$ although they don’t as yet offer software to help dial them in, which is not necessary but can be very helpful.  Arendal subs have also gotten good reviews and seem to offer high value, and I’d also consider Paradigm and Perlisten.  B&W is overpriced relative to alternatives IMO, and JL Audio has had significant reliability issues that may be more prevalent in their lower-level subs but worth checking before going that route.  I want my subs to hit an honest 20Hz because there’s important information down there that improves sense of space/soundstage, and to get there with REL you gotta spend big bucks relative to these other excellent alternatives.  If it’s me I’d go with a Swarm from Audiokinesis, but if that’s not possible I’d go with a pair of 15” Rythmiks and take the thousands you’ll save and upgrade other equipment or go on a nice vacation.  Hope this helps, and best of luck. 

Welcome to the dark side. The answer to your question is, it depends on what your goals are, what you listen to and how loud you listen. If you prefer aggressive bass driven music, like blues, rock, EDM, and even modern country, specially at higher volume levels, bigger will be better. Be warned. A good subwoofer system will expose room node issues, which is why more is better than less, up to four.

I would read this before doing anything.

https://www.acousticsciences.com/wp-content/uploads/2024/11/Soundoctor-Barry-Ober-sub-integration.pdf

A great subwoofer system can be immensely rewarding, they can also be immensely frustrating. I have been using subs for decades and much to my chagrin, I’m still learning.

Two subs are always better than one. They extend the soundstage. 

This may be true, but for me it is subordinate to them helping with room modes. Two is good, three is great, four is...marginally better.

Helpful:
https://blog.teufelaudio.com/what-are-room-modes/
https://forum.audiogon.com/discussions/subwoofers-and-phase-question-for-you-sub-experts
 

I added 4 subs and it was the best bang for buck ever.