How many subs?

@bilateral  Considering your living arrangement concerns the -6dB sub-bass speaker suggestion is understandable...but no. 

A subwoofer with a modicum of frequency response control would be a far greater advantage while maintaining the ability to go low if or when the opportunity or circumstances change. Ultimately, a subwoofer with customizable presets would allow you situational flexibility and convenience. 

I purchased my first subwoofer in 1967. Living in wood framed and floored ranch homes most of my life and annoyed by the travel of those filtered bass frequencies. While my little studio is substantially treated the HT room isn't and only first reflections are lightly addressed in the two channel room. I'm not a fan of closed in treated rooms. 

My two twelve inch subwoofers rest on simple to build DIY dollies. I used these soft rubber plate casters attached to twelve layer Baltic Birch (any 3/4" plywood) followed by old Boogie board closed cell foam and topped with four inch heavy latex, foam store foam.            

https://www.castercity.com/product/5-super-soft-gray-soft-rubber-plate-swivel-caster-3/

Decoupling, suspending and the twelve inch hight increase required resetting their settings. The resulting improvement in bass presentation was very noticeable. Unknowingly, the transmission of resonant frequencies throughout the dwelling structure were stunningly reduced. No more rattling and buzzing, gone, except for preset #6.   

@m-db

The Rel subs have to be on the floor - another disadvantage.  I tried platforms and it really sounded much worse.

Subs that can be decoupled from the floor, with proper bass extension, and ability to knock some lowest frequencies out of the mix when playing particularly annoying bass heavy music, looks like a good solution.

The sound track to Blade Runner - CD (probably all formats) has some excellent bass moments.  It would be great to hear that with fully capable subs.

I like that there is a preset - 6 - that is too much.

You always need a preset that risks causing an earthquake.

Iam using three 2 pioneer sub modified by a guy who sold Teajay the pioneer modified. I heard them at his house pre pandemic with the ps 12. One sub Iam using  denon. I used three because of my room configuration.Atmasphere is right audiokinesis does has swarm set up, that I considered but I decided to do 3.

@jayctoy any number of subs is good - I just got the 3 sub system going with some outrageous hardware. It sounds fantastic.  Not the lowest bass - I don't want that (yet).

Delays with a Drive Rack. Low pass filter for the right subs with a CR1 - nothing spared.

I need some tweaks to get it right.  When you hear a Tyrannosaurus Rex on S/510s - you will run.

Everyone will be pleased that the experimentation with delays for subs 2 and 3 is done and it is a fail.  It takes a few sessions and many tweaks of levels to hear what is happening.  To get the sound so subs 2 and 3 don't noticeably interfere with the main speakers is a very low level.  I turn off output 1 on my amp and listen to only output 2 which feeds the 2 delayed subs.  Really not a lot happening.  Not worth it.