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

@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.