I bought 4 subwoofers and I'm absolutely not doing a DBA! Hah!

You probably know this but just in case and just a suggestion: check with the driver manufacturer’s specs on the cabinet’s internal volume? 

@iseland "Plateamp": I moved my "plate amp'' out of the sub box into a separate housing. Its still a plate amp (components mounted on a plate), just a 'plate am in a different housing'.

Given the same space for the sub, I wanted that extra volume for the driver, and it allows me to swap out amps/crossovers, or use the 'housing amp' to drive other subs. 

Its the 2nd post I read today that mentions "enlarged soundstage"
and "air" in the context of woofer/subwoofer ("2 subs provide enlarge soundstage and more air"). 

I never brought the low frequencies in context with 'air' and 'soundstage'. 

Should I start thinking adding a 2nd sub (2 smaller ones instead of 1 larger)?

Me personally,  there are plenty of excellent amps without plates in already made boxes I would buy instead of making my own.

And don't get me wrong,  I love my hypex plate amp ! But if I  didn't had the need for convenient space saving solution I would buy a typical all in 1 already made box .

@kraftwerkturbo It's a real thing.  I suspect much has to do with the appropriate high pass filter though.  Several a'goners who I have convinced to try raising the HP point from 40 to 80 Hz or higher have reported the improvements along those lines.

I have had this happen with 1 sub.  It's not about how many subs but about the sub to main speaker integration, but if you want to add subs, who am I to stop you?

Another benefit of using a subwoofer amp (often done as 'plate amps'), and the very first reason I used mine (aside from beying ICE design, well working/sounding, powerful): The amp came with adjustable crossover frequency and volume control (unlike my other existing 'power amps'). So using it was a given; putting it NOT into the tight space (sealed) box of the sub was just gravy (more usable volume for given exterior dimensions)  AND easier to acomplish. 

But I guess the adjustable crossover frequency is also part of most receivers; in my case, I need to select 'small' speaker first, then I can select high pass and low pass frequencies. 

@erik_squires  talking about 'air' and 'enlarged soundstage' with respect to subwoofers: "It's not about how many subs but about the sub to main speaker integration": how would you go about better 'intergration'? 

@kraftwerkturbo 

The issue is getting smooth bass and seamless transition from sub to main speakers.  Personally I've solved that issue with a combination of floor to ceiling bass traps in the corners and EQ.

Others rely on multiple subs.

Of course the issue of good speaker and listener placement also matter.  The AM Acoustics room mode simulator is a good place to start.

The reality is that subs have 2 problems.  The room and the main speakers.  You must blend it into both at the same time and I know of no easy way to get around measurement.  The Distributed Bass Array is not to my mind easy, cheap or something everyone can implement, but it has it's fans for a reason.

Reading the comments about amps I think the average reader doesn't unerstand that the amps I'm using are 3 channels of amplification plus digital signal processing.

While similar to the average subwoofer amp you'll find screwed onto the back of all active subwoofers the Hypex multi channel amplifiers do not have simple controls.  There's no knob to set the low pass or high pass or volume.  Everyth8ing is done by the crossover designer (me) with their application. 

In essence if you can imagine a miniDSP EQ and crossover plus several amplifiers built into a device that shares a single power cord you will have a better understanding.  These amps and others like them are the heart and soul of modern active loudspeakers.

Ahem, $125,000 a pair for these.

Lordy.

@erik_squires   ...a pittance...*yawn*  Whose, so I can wire the wherewithal...

Interesting material mix, with novel appearances....one could say 'brutalist' and be on target....
The wall graphic is a bendy bunch....*L*

DSP EQ and crossover plus several amplifiers  

Would 3 cords be OK for just those?....Full Minimalist...
...not counting the 'peripherals'.....sources, basically. ;)