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

Hey, it is just my experience and YMMV.
For me its much more important, easier and may be cheaper to address issues like standing waves in bass frequencies, early reflections and reverberance in highs, jitter (if you listen to digital), speaket positioning and toe in, listening position, AC power conditioning, etc.

And yes!
Audio improvement its a neverending story!!! Its an addiction!!  :-)

So when ya ordering that sub plga? ;))

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