Why the fascination with subwoofers?

Few "full range" speaker go much lower that 40hz. That is a whole octave missing. And a subwoofer designed for home theatre is not what most audiophiles want.  They want one that plays music convincingly.  In fact home theatre speakers are not what audiophiles seek either.  Now there are true full range speakers, but they are typically very expensive and quite large.   Often a good subwoofer paired with high quality monitors will be a good option, even preferred by some. 

low E on a 4 string bass is 41hz.  A lot of speakers can't even go flat to 40hz.  Low B on a 5 string bass is 31hz.  Lowest frequency on a piano is 27.5 hz. 

Ive never hooked up subs and dont feel I’m missing anything. Different speakers bring different results and it just seems somehow obvious when any part of the sound spectrum is not quite right and time to move on to another speaker. Had some Monitor Audio one time and dont get the attraction there. In my system at the time at least. Thin and a bit etched. My Harbeths are all around a good sounding speaker. I suppose for home theater some may want that all enveloping sound of the lower frequencies ... I never got into home theatre,, Just doesnt interest me but can understand why some do.  @kink56   what you are saying  does makes sense and I have to admit I've always wanted to check it out.   Dont know that I will anytime soon.  

I have a pair of 12" sealed subs playing mono and been wondering about adding one or two more with the goal of flattening at the main seat. Would love to know your take on this.

I'd give it a shot. I don't use the Swarm but if I had it to do over that's the route I'd likely take. Duke is working on a sub system for me though- one that is integrated into a coffee table. It will be used to break up the standing wave in my room.