How many subs?

@bilateral  I don't think there is a standard for Digital Signal Processing. Its not the answer for you anyway.

I'm aware that some companies rely on matching their products to speaker brands. Of course this is absurd. Actual subwoofers are adjusted, not matched to the speakers. 

-6dB@20Hz is hardly, if any, 20Hz output at all. 

You're looking for that delicate extra low +2dB@20Hz feeling coming from many modern recordings. If a dealer did that matchmaking, demand a refund or a store credit toward some actual subwoofers. You'll be amazed.  Take care,

to be safe, build the back wall from subs. You have never heard anyone say

I built a wall of subwoofers and I so wish I hadn't

Cheers everyone.

Everyone knows I'm going to still give the delays another go - even just to better understand I don't like it.

From my engineer:

"The method I used for setting up the three subs wasn’t with all subs at the same loudness (in my opinion it isn't correct due to different distances from the listener and other factors...). I used a method, which slightly varies the dB levels between each sub to achieve a smoother in-room response. This approach focuses on reducing peaks and nulls caused by room modes while maintaining overall balance."

The original subs I chose were lesser Rel subs.  I gave instructions to where I bought them is I cannot have the deepest bass due to neighbours.  I am already aware that they can hear what is happening.  I want to have some hope of being able to play music.

It is Rel that paired their choice with the speakers.  The S/510s are needed to keep up with the 805s.  They insisted that TX7s or whatever I chose weren't good enough.

I installed just over 3 inches of acoustic treatment along the party wall in the hallway, lounge and kitchen - 4 layers, including 3 layers in "acoustic-board" specifically designed to reduce vibration.  With the double brick, I estimate (and because I can no longer hear the neighbours) I might have achieved close to Rw + Ctr of 60 dB.  But that won't stop low frequencies - nothing can completely stop them unless we go with "in space, no-one hears your subs."

The room is separated from the hallway by a brick wall with plaster on each side.  The front window got a double glazing upgrade using a material that isn't glass and with a large distance between them.  Then theatre blacks - 95% wool curtains.  This is the front wall.

Acoustic treatment (above ~200Hz) around all walls which surprised me how much it quietened noise from outside.

With the acoustic door - that room is sealed tight.  Acoustically and thermally.

Even with all this - I can't have subs that properly do 20 Hz.

@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.