Single vs. Dual Subs

Strictly interested in confining the discussion to your pre-determined conclusion. Wonderful. But you get the answer you weren't looking for anyway. Each additional sub, by being in a different location adds to the number and location of bass modes. The result is that for each additional sub the bass becomes smoother, and smoother bass is faster cleaner more articulate bass. It also improves extension, slam, and dynamic headroom. The man who wrote the paper discovering the distributed bass array approach you are so intent on not talking about even reduced it to a mathematical formula expressing the improvement of each addition sub.

The single greatest development in high end audio in a generation, but you don't want to talk about it. What a shame. Oh well too late now.

Your single sub by the way never can be "dialed in perfectly" but in order to understand why you will have to first permit talking about that to which your mind is currently closed to considering.

Wouldn't it be wonderful if we could just dictate to the world how things are? Maybe. But it would not be the world we live in now, would it?

You can obtain satisfactory results with one sub, but to achieve really great integration you also you need; lots of time, and a way to equalize and tailor the bass frequency response to correlate to your listening position. I have found great results by using an old Velodyne SMS 1 Sub equalizer. It allows you to hook up a monitor and has a long enough microphone cable to setup at your listening position to achieve exceptional results. By adjusting EQ parameters on the Velodyne EQ, you can see in real time what effect  your settings will have at the listening position displayed on the monitor. It is easy to use if you under stand crossover settings, slope, phase, -+20db settings for frequencies of 20-120hz, and a few other settings that I forget. It does allow up to two subs to be hooked up. 

One Sub does not have the power to load most rooms down to 20 Hz at least up to 15 inches. An single 18" sub might get you there. Two 12's is the minimum. Two 15's even better.

Speaker frequency response is taken near field usually at one meter from the speaker. 4 meters back in a 16 X 30 foot room is a much different story. In order to get the right "feel" you need a rising response from 100 Hz down to 18 Hz. That takes big drivers and a lot of power. Two subs will also smooth out the nodal behavior in the room. Without proper low end you will never feel as if you are at a live performance.

“What do two subs add to the music presentation that a single dialed in sub is missing provided that single sub is dialed in perfectly for the listening position?”

When you add a second subwoofer, most of the peaks and dips get canceled out by the second sub. Two or more subwoofers also help in reducing low-frequency distortion and offer improved dynamics over a single subwoofer. This is a simple function of having more radiating area, plus more power on tap.

The realism and imaging you experience from two subs over one is unmistakable.

OP, I appreciated your question because my room cannot accommodate a SWARM. When I move into a new room, I will be able to try it. Your question is a logical one for someone like me, and there's no reason you should be berated for limiting the question. After all, you said "at the moment" and "caveat" to try to forestall the kind of denigration you (alas, inevitably) received. 

I like the answer from @lalitk  for it's directness and simplicity.

I found a some good information, here: http://www.avrant.com/av-rant-737-interview-with-todd-welti-on-subwoofers/