Potential subwoofer purchase-a few Q’s

Much better.  Seriously though, I'm not sure how to quantify it.  Two subs can be great for movies if you just want to have that visceral feel of the bass effects in the movie's soundtrack, but for music having four subs is a subtle and at the same time dramatic difference.  Subtle in the sense that they don't draw attention to themselves, dramatic in that the bass fills the room (not just one or two sweet spots), and is immersive.  I'm sure some of our more eloquent members can describe it better, but I'll summarize with this - if you have room and budget, do it.

Rythmik, IMHO makes, OK subs. Find one of the Martin Logan, "Descent", model subs and listen to it. It will actually change the way you think about subs to begin with.  I tried them all. And the Descent subs are soo far past everyone else it is ridiculous. Maybe because they use "Real" servo correction and not "Feedback loop calculations"? That is just the cheap way to attempt accuracy. Better than nothing but not so great when compared to the real thing. The 7500Watt amp doesn't hurt either. I use three Descents in my main system.

Millercarbon, now you know I use 4 subwoofers but with a smaller system two subs can work fine but two subs is the minimum. I used a two sub system for years until I graduated to line source speakers then two was not enough to match the power projection of the speakers. I did not know the acoustical benefit of multiple subs in terms of minimizing standing waves. 

Jollygreen, the Descent subs are long discontinued. The New ones are called Balance Force. They are no longer servo driven and room control is an optional extra. The balanced force design is brilliant. It does keep the cabinet from shaking but contrary to the marketing does not reduce cabinet resonance. A very stiff enclosure made from acoustically inert material does that. The amplifiers are relatively standard class D affairs. They are inexpensive and efficient but I prefer AB amps with damping factor over 500 sound wise, which unfortunately means passive subs and out board amps but 40 years ago that was all we had.  

Millercarbon, now you know I use 4 subwoofers but with a smaller system two subs can work fine but two subs is the minimum. I used a two sub system for years until I graduated to line source speakers then two was not enough to match the power projection of the speakers. I did not know the acoustical benefit of multiple subs in terms of minimizing standing waves.

Yeah that's what got my attention in the first place. One sub creates standing waves. Every sub. Every room. Its physics. We know this. The epiphone was to figure out the answer is lots of subs. With more subs in more locations they no longer have to be very big or powerful because there's so many. And being in so many different locations the modes or standing waves they create are all over the place. So instead of a few really big standing waves you get to where they are so small and so many its almost like no standing waves at all. Its just brilliant, creative, genius! 

I recently had a guest over and he is one of those guys who has been to a lot of shows and heard a lot of really high end systems and even he was impressed. Which is freaking amazing considering my subs cost about $3k all together and my whole system is probably about the sales tax on a lot of them nowadays. Yet his comment was, " I heard fabulous clarity and fat, full soundstage. I’m not just saying. I heard and felt it!"

Note the "felt it" part. And what he didn't email but did say in person is the bass is totally integrated and at one with the overall presentation. In other words the "fabulous clarity" includes even the deepest bass.

This is because Duke and Geddes, et al, are right. Smooth bass is fast bass and the more subs the smoother it gets.

The physics is the same regardless of the room or the system. The only thing I can see changing if the system or the room gets a lot smaller is the subs won't need to be as big or powerful. But they will need to be many.