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

You have all lost me.  Guess I need to get a sound engineering degree to understand.  Wonder how you all learned this?  

Dave
I already bought one once and never again!!   :-) 
Larry
Well, even Im not a tech expert AT ALL, it took me about a couple of years, a lot of hours and mostly some not small cash to learn it the hard way and I think the only way.... Readind, buying, connecting, selling, tweaking, tweaking again, listening, more reading, etc.
If you like this hobby like me, you will do it without getting tired!!
Cheers to both of you

@plga - thanks for sharing your experience.  

I've often wondered if it's worth the trouble for me to pursue the bottom octave knowing that it will be difficult to get right.  Sounds like it can be done but be prepared for a daunting task.

For now, I'm going to keep it simple.  

Hello Mike
I really dont know on what kind of music you will miss that bottom octave.
I think, as I guess all audiophiles do, that our biggest goal is to have a system that reproduces the music as close as possible to the real thing and I think that bottom octave, if you have it on some music, should not be your priority. I dont feel the music on my system lacking some Kind of base, quite the opposite, sometimes I feel I over boosted the bass with Dirac on some songs. I must confess, I like the music with weight, body and texture, I love a well rendered bass in the music, may be with a little excess of it.
You can ask some experts (Im not one) and Im sure they all will agree that there are many other issues to spend time and money on before getting subs.
If, and only if, you had bookshelf speakers, in most cases I agree that the bottom end will sound not too powerful or you will need a good muscle amp to get dynamics and proper bass as bookshelf speakers in most cases have very low sensitivity and they are not easy to drive.
If someone has bookshelf speakers in a medium to big room, I would advice to sell them and get the best floorstanders you can afford instead of buying one or more subs.
I would only consider buying subs if, after having a very nice and voiced 2.0 system, you dont get a convincing bass.
Also dont get confused about bass, because many of us have been used to consider excessive bass as a natural and good sounding one and it is not correct! Most of us grew up in Discoteques and going to concerts and thats not how unamplified live music sounds.
Listen some guitar, drums, sax, trompet, etc unamplified, they dont have excessive bass, they have texture, depth, dynamics, etc. And IMHO to get that you have to address some more important issues like I said.
Anyone thinking about buying a sub before getting the best system they can afford and voicing it, should first manage to listen to a very good and voiced hi end system. It will change his mind.

Hey Sparky, I’ve owned Wilson 6’s, 7’s, Dynaudio C4’s, B&W 802D3’s etc and Reference hear from ARC, Levinson, Krell etc..  Been there and done the whole no sub thing.  Short of those types of systems and great rooms (insert plga) a sub adds texture, spaciousness and warmth.  It is not to blast more bass.  Adding room correction electronics to a system, IMO adds extra artifacts and strays from the original musical content.  Talk about using a computer to deal with a problem an abacus could solve...jeez!  Placing a sub is hard if your hard of hearing.