Single vs. Dual Subs

“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/

A single sub can do the job but you MUST find that "perfect spot" in your room. Incorrect placement will sound horrible.

I have two JL audio E112s both are high-passed at 60hz but no dsp. 
Over the last few months both broke (bad volume pots) and needed repaired so I had one sub at a time as I rotated then out for repair. 
During this time I optimized the placement with one sub (large dedicated room). After a lot of messing around I got one sub to sound fine but the volume level adjustment was picky and it never felt just right. I assumed this is because there were some lumps in the bass. Some songs seemed light and some seemed heavy. 
After a few months I got both subs fixed. Dropped them into their normal spot and boom everything locked into place and sounds much better. No more chasing sub volume adjustments. With two subs placement is much more flexible too. 

@lalitk,
+2