Why the fascination with subwoofers?

Great subject. Right now I am in the process of doing room treatment and my bass sounds great. But I will still add 2 subs. I have demoed a few in my room and they really open things up.  My mains have excellent base, but the lowest of frequencies are missed and I want them also. Thanks for all the info, helps alot.
++++on adding subs.

I have one of my subs near a wall and under a window in my listening room, and when I want sound on the deck I put a pair of speakers in the windows with that sub. The deck specific speakers (old KEF Q10s...great sounding things) are powered by a separate amp (a trusty 100 watt Adcom) getting its signal from the main preamp, and the sub's signal is from the indoor mains little tube amp so I have to turn the window sub up a little...I have a volume control on the outdoor amp as it's way hotter than the indoor speaker's amp...all of this works very well, and unlike mapman I get my deck bass goin' which is good for the plants and lets my neighbors know I'm having more fun than they are. 

Full range cabinets are going to be rather large.  It can be much more convenient to go with smaller speakers plus a sub in a less conspicuous spot in the room. 

My understanding is that having the deep bass coming from more than one speaker invites phase cancellation.  

I assume that taking the deep bass out of the speaker handling the low mids and bass frequencies will help to make the audio clearer. 

@atmasphere 

Hello Ralph.

I see you use a SWARM system, so the sub woofer frequencies play in mono on a distributed basis. Similar to Earl Geddes approach/theory. Geddes argues for 3 to 4 subs playing the same signal to flatten frequency response at different locations in the room. I was wondering if you are using this approach for the same goal, for flattening at the main listener seat, or other?

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.

Regards 
Horacio

     Since bass sound waves behave so differently in any room versus midrange/treble sound waves in the same room and good bass performance is more difficult to achieve in any room versus midrange/treble performance in the same room, I'd suggest it makes sense to consider our system as 2 systems,a bass system and a midrange/treble system, and it's logical to deploy and optimize the bass system prior to deploying and optimizing the midrange/treble system.
     Starting from scratch, the best method for deploying and optimizing bass performance in any room that I'm aware of is a 4-sub distributed bass array system.  
     For those already owning high quality full range floor standing speakers that are capable of outputting accurate deep bass down to at least 20 Hz, however, I believe it may be the exception to the general strategy of getting the bass performance optimized first.  This is because these types of high quality speakers are unique since each speaker, in effect, already contains a high quality sub due to the exceptionally good deep bass extension of each speaker's bass section.   
     In these scenarios, I would suggest first positioning the floor standing  speakers to optimize the midrange/treble response as well as the stereo imaging at the dedicated listening seat.  Once this is done, the positions of the first 2 'subs' in your custom 4-sub dba system are established.  From here, it's just a matter of buying 2 more subs and positioning them in the room so that bass response is optimized at the dedicated listening seat. I even believe the 2 additional subs don't need to be of the same quality level or size of the main speakers' bass sections for the dba concept to work effectively. 
     This method is a bit of a compromise from a complete Swarm type dba system but I believe it will provide very similar results.  I believe this because the main dynamics involved in the very effective dba concept would still exist; 4 asymmetrically positioned subs with each creating bass room modes.  At the listening seat, the brain processes the multiple room bass modes (bass peaks and dips) by summing and averaging them which results in the bass being almost magically perceived as detailed, natural and smooth. 
     Voila, psycho acoustic principles at work. The dba concept absolutely produces the best bass performance I've ever had in my room and system.  I was initially very skeptical and I know just my words will likely convince very few but I also know even a brief demo would convince everyone.

Tim