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

Well
Ive used a sub with a pair of bookshelf speakers.
Then I change them for a nice pair of floorstanders and I couldnt make them work well with the sub as it only blurred and masked the sound of the floorstanders, A LOT. 
My opinion is that it is VERY difficult to get a the same frequency integration of a nice pair of floorstanders with bookshelf speakers and subs. 
Today I have a high end pair of floorstanders, with some room correction and Dirac Room Correction software and the bass is oustanding, incredible, and with out blurring or affecting the sound of mids and highs. 
If I were you, I would sell the bookshelf speakers and buy a nice pair of floorstanders. Otherwise you will get into something pretty hard to resolve and you will very likely spend more money and time on the process.
Then I would spend money upgrading the source and the power amp and then time voicing the system to the room.
Im sure it will be much more productive. 

Unless of course you have and immense room, wich its not most cases. 
Mine is 14' wide, 28' long and 9' tall. 
Trust me, dont waste your time and money dealing with subs, if its stereo music what you have your system for.
Movies with home theater is another story and hobby. 

Not true plga.

Well, its only my experience.
But, its true that integrate a sub, or a pair of subs, its quite complicated and, to my taste, a nice pair of floorstanders, with a nice system behind and all of them voiced to the room, can sure give you all you need in music.
Just finding the right spot for your speakers (with out a sub) and your listening position its a big challenge. Not to mention taking care of all the rest issues to correctly voice the system to the room.
Getting that done WELL, but with subs, IMHO is for experts. If you are not one, you would need A LOT of time and patience to get it right.

I cannot agree more with erik_squires when he said "It’s not the tech is bad, it’s the complications in the setup that I’ve so often heard as poor".
Not to mention the space the subs will need in your room, powering them, cabling, etc etc. 
For me, not worth it.