One Sub or Two?

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.