Will a subwoofer add depth and clarity to my system, or just bass?

Ya puts the speakers and the sub in da room and ya moves em around till day sounds da best!  Badabing....hopefully not badaboom :))

Hi guys,
I´ve been styuding about adding a pair of subs to my system.... you and Jim Smith got me convinced!

Although I wouldnt recomend bookshelf speakers with subs instead of a good pair of floorstanders, it seems that a pair of subs could improve quite a lot the sound of a full range stereo speakers. We'll see!!

I live in Argentina and it's much more expensive for me (about 50 to 100% more) to buy gear and impossible to try them home and have the chance to send it back to a dealer if I don't like it. So, I have to buy blindly, following the advice of others.

Wich subs would you recomend under this limitations:

1- My budget is about 1k top for each sub.
 
2- I would prefer not big subs (due to room and decor limitations). I only have two squares about 2.5' x 2.5' each, between the speakers and their respective side walls.

3- They have to be able to reach at least 25 Hz.

4- I've been advised to buy ones that can be connected to the output of the power amp, not to the preamp, to have better results.

I think the SVS SB-2000 could be a good choice, but it cannot be connected to the power amp, as I understand. How much does this issue affect the sound?  I have to mention that the distance between my amp and preamp to the possible locations of the subs is about 16/18' the longest and one of the cables for the subs (if a dedicated ones are needed) will have to share the same pipe as one speaker cable.

What about cheaper brands like Klipsch or Polk Audio?

I've had one Polk Audio PSW125 with a pair of bookshelf speakers and then with a pair of floorstanders and I didnt like the results AT ALL. Of course I didn't had it correctly voiced and it was only one sub, so I cannot blame it for the bad results. 

Thank you in advance!

Ive just found out the Kef Kube series. For my room, I guess a pair of Kubes 10b should work very well.

It's relatively small (easier to move around to reach the best location), reaches 24 Hz, looks nice on the outside, accepts speaker connection, it costs $600 each and it has good reviews. 

Has anyone tried them?

millercarbon,
    Excellent, I'm glad you've got the room gain factor covered with Duke's help.

Duke,
    I think I might have gotten a bit lucky when I setup my 4 Debra subs in my room. I took all the port plugs out at initial setup with the intention of auditioning the system as all 4 running as ported subs first. If I didn't think it sounded very good, my plan was to plug them all and audition the system again as all 4 running as sealed subs to determine if this performed better. If I wasn't pleased with all subs ported or sealed, I figured I could try various combinations of ported and sealed.
    The reason I think I got a bit lucky is because it sounded almost unbelievably good with the first content my friend and I decided to audition it with, a bluray disc of the movie Fury.  
    The first real test came with a preview of the movie Whiplash on this disc that included a drum solo that sounded so amazingly accurate and lifelike that it actually left my friend and I just looking at each other in stunned disbelief.  
    There was an effortless quality to the bass while at the same time a sense of unlimited capacity to the dynamics which I'd only previously experienced with live music.The quality of the bass, as the tempo of the drum solo began slowly at a normal volume but kept gradually building in tempo and volume, was so pitch and tone perfect, accurate and detailed that there was no hint of betrayal in the solid and stable sound stage illusion that there was a guy expertly playing a full drum kit positioned dead center along my living room's front wall. The only accurate way we were able to describe this high a quality level of reproduction was to call it state of the art.  I've been enjoying the Debra system's sota bass response for about 3 yrs now.  I thought you might find it useful to know that I still have all subs configured as ported, with all in mono and powered by the single Dayton amp with the crossover freq. typically set at 40 Hz,no polarity inverted on any sub and my main speakers (with useful bass only to about 36 Hz) running full-range.  The integration between the deep bass of the Debra system with the upper bass to treble of my  large Magnepan panels is seamless. 
      I know the above may sound like a bunch of hyperbole to many reading this because I would probably be highly skeptical reading this good of a review myself if I didn't hear it for myself. I completely agree with the thought of trying to audition a properly setup DBA (distributed bass array) system prior to buying a complete Audio Kinesis DBA system for $2,800 like I did or building your own custom DBA system like millercarbon is currently doing for possibly less money.
      I could probably write a book about how great the AK Debra system works in my system/room and convince nobody but I could probably also give a brief audition of it and convince anyone.
      Since my intention is just to spread the word to fellow music and ht enthusiasts about how exceptionally well DBAs actually perform, however, I have had some concerns about whether the DBA concept really is effective in virtually any given room. If I recall correctly from reading Earl Geddes's White Paper on this subject, I believe he even uses the phrase "in any given room" but I'm going to recheck just to be certain.   I definitely know it works in my room and fairly certain it also did in the Absolute Sound reviewer's room since his review description linked below of the Swarm DBA system in his room so closely resembles my experiences of the Debra DBA system in my room.

www.theabsolutesound.com/articles/audiokinesis-swarm-subwoofer-system/

     As I think you're aware, I was convinced to buy the Debra system by your colleague James Romeyn's generous offer of a full refund option after a 4 week in-home trial period.  Of course, it didn't take long to not only know I wouldn't be returning it but that I'd never need to buy another sub or bass system in my lifetime.  I now have sota bass response but the Debra is still the only DBA I've ever personally setup and the only DBA I've ever personally even listened to in my system and room or any system and room.
     I prefer to make statements and offer opinions that are accurate, truthful and not exaggerated. I respect the obvious knowledge and experience you and James both possess concerning DBA systems and just have a couple of clarifying questions.    
    Are you or James aware of any DBA systems not performing well in any room or system? 
    Are either of you aware of any DBA systems not integrating well with any particular brand, model or type of speakers?

Thanks,
  Tim