How does adding a second subwoofer improve your systems SQ?

EG: Just listen to the second last track "It’s A Miracle" on Roger Waters "Amused to Death" at the end there is a kettle drum roll that rolls around the room on the ground from right to left and back again. If you had just one sub it would ruin that mesmerizing effect totally.

This statement is misleading. I get exactly that effect using only one sub in my bedroom system.


The issue is that below about 80Hz in most rooms, the ear cannot distinguish where the bass is coming from because it cannot detect a sound until the entire waveform has passed by the ear. In most rooms at about 80Hz, the waveform is so long that it has bounced off of the rear wall and is already passing the ear in the other direction before the waveform has completely passed the ear on its way to that rear wall!

It is harmonics of bass instruments like the string bass, bass drum, tympani and the like that convince you that the sound is emanating from in front of you and whether it is to the left or the right. So a single sub works, 2 subs better, 3 better than that and beyond 4 you're at the point of diminishing return. It is important however to make sure that bass information is coming from both main channels to the sub system!


However this is only true if the sub or subs do not attract attention to themselves and to do that they can't put out anything above about 80Hz. If they do, now the subs have to be time-aligned with your main speakers and its a royal pain in the rear and at that point a DBA doesn't work. So as a general rule, make sure your sub never ever goes over 80Hz.

Let me give you a practical example-

In my 2-channel setup I basically have only two small areas where my left and right subwoofers will fit- so positional flexibility is very limited.

I recently measured the response of each sub during my integration process.  The left sub had a dropoff below 30 Hz and small peaks above 60 Hz.  The right sub was flat down to 20Hz but lacking some impact.  
When both playing together the response is smooth and flat at my listening spot, sounds exciting, deep, dynamic and smooth.  
Having two subs is a great advantage to achieving a balanced sound if you have limited room to position them.     

I get exactly that effect using only one sub in my bedroom system.

You are getting a just sample of what’s going on down there, you are drawing conclusions about the Roger Waters track with regards to hearing it with two L and R subs, it’s obvious. Because if you did you wouldn’t be saying what you just said.

I done the a/b with 1 mono vs 2 in stereo. And my subs are at 50hz 4th order, and the difference is huge.
Anyone who mono’s their bass below 100hz is ruining many albums that have been recorded in stereo down there.
  
It may not matter to the vinyl crew so much because they’re lucky to get 10db of channel separation down that low from their phono cartridges, so they are basically listening to mono anyway.

Good article on speakers  by Floyd Toole, close to the end there’s a section how to get great sound from a single subwoofer. As long as you’re only interested in one listening position.
https://www.harman.com/Documents/AudioScience_0.pdf

Avanti,
Having two subs is a great advantage to achieving a balanced sound if you have limited room to position them.    

This is what I was waiting for. Unless you have a dedicated audio room, I don't, there are definite limitations to most all rooms. My system is in my living room. I simply can't put subwoofers where they might sound best. I'm confined to placing my sub next to one of my main speakers. Avanti, so your thesis is, buy a second sub and place next to the other speaker and enjoy the music.