How many subs?

@bilateral No need for any delay in such a small room! If your 'acoustic engineer' thinks otherwise, fire him- he's doing you no favors.

Here are some facts to consider: at 80Hz, the waveform is 14 feet long. It takes a full cycle to pass the ear before the ear can tell its there; it takes a few more to tell what the note actually is.

By this time the bass has bounced all over your room. This means the bass is 100% reverberant.

For that reason you can use a mono bass signal in most rooms except those that are very large. The subs need not be anywhere near the main speakers as long as their output is kept below 80Hz so as not to attract attention to themselves.

The problem you run into in a room like yours is something called a 'standing wave', which can cause bass to cancel. Cancellation can't be corrected by bass traps or digital room correction since that would simply ask for more amplifier power which would get cancelled. To break up the standing wave you need multiple subs which are asymmetrically placed about the room. This isn't the sort of thing that 'takes hours'.

For this you need 4 subs to be the most effective. This technique is called a 'Distributed Bass Array'. It will breakup any standing waves and will take care of 95% of the room's bass problems. Only after they are installed should you try bass traps and/or room correction, which can do the other 5%.

End of 'facts to be considered'; now for the anecdotes:

My speakers at home are flat to 20Hz. I had a significant standing wave. So I needed two subs to break it up. One is to the left of my listening position and the other is behind me and to the right. This worked a treat and literally took a few minutes to set up.

Audiokinesis makes a sub called the Swarm for this purpose. Its designed to be set as close to the wall as you can get it. In my room, I have the drivers actually facing the wall so as to be certain its inside the room boundary effect. By so doing, the Swarm subs are also flat to 20Hz. A single, inexpensive subwoofer amplifier recommended by Audiokinesis drives the two subs.

For my system I do use 4 JL-F-113 subs. My room size is 17’ x 25’.

The subs have a calibration mic that I use to set up each separately in the chosen location.

After all are calibrated, I use 45hz as the crossover point. Everything blends beautifully and extended.

Though each sub has a delay feature I don’t use it. Try to not make this too complicated.

ozzy

@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.