One Sub or Two?

In a choice between one or two, I’d say two. I have two JL’s, but wish I had four🤷‍♂️

It may not make intuitive sense at all but two subs produce a lot better and a lot more control over bass. I know people always say that bass is not directional -i.e. you can't always tell where the bass is coming from but with only one sub it is likely you can detect the source of the bass you are hearing. The goal is truly making it difficult if not impossible to determine where the bass is coming from -i.e. being able to blend it with the rest of your audio signal and 2 subs seems to make it easier to do that. It is likely you will spend some time determining exactly how to position both subs but my experience is alternate corners of the room can work very well and you can play with adjusting the exact direction for the subs from there. As to which subs to select, I would suggest self powered subs and my choice was 2 JL Audio Fathem F-113 subs.

@nankaroad: 

In a 40'Lx30'Wx12'H room with B&W 801 D4 speakers no less than two 18'' subwoofers. Subwoofer integration is key see here and for bass myths see here.

Mike

My B&W 803 D3 speakers sound improved significantly after adding two JL Audio subs positioned beside each speaker with the plane of the subs and the plane of the speakers equidistant from the listening seat in my 14x15' room.

At 15Hz-28kHz (+/-3dB) your 801's low frequency capabilities produce substantially lower output than any six of REL model sub-bass speakers, all of which typically begin a dramatic -6dB low frequency roll off in the mid 30Hz region. 

While your 801's clearly outperform REL they're probably located in the middle of your rooms null which can suck their full range low frequency output into thin air. 

To enjoy the subtle extra low frequency presentation provided by any of the suggested -3dB actual subwoofers and the ability to go +dB in the 15-25Hz region the subwoofer will require optimum room positioning typically along sidewalls.       

https://www.audioholics.com/home-theater-connection/crawling-for-bass-subwoofer-placement

While all rooms are different you only need to mark and map any room once using one sub at the listening position. Download and use a low frequency tone cadence.

I suggest using well made and inexpensive custom length subwoofer interconnects from Blue Jeans or Monoprice Cable.

Welcome to the deep end, Rock on.