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

Well, IMHO, its very difficult to deal with standing waves in rooms with just one pair of speakers.
Ive tried to make a pair of floorstanders work with a sub and I couldnt do it, nor did I have the patience to get it done. It was too difficult. It only made the speakers sound much worse and provided me a LOT of standing waves all accross the room. It blurred the sound of the speakers.
On the other hand, Ive manage to get a very nice system, without subs, that provides me an "incredible" musical experience. It has bass (a lot), depth, 3D sound, tone, dinamics, layering, detail, etc etc.
If you consider the money you have to spend on more than one sub, the time to correctly position them (the possible locations and orientation angles are a LOT), then the time to voice them, the space they need, cabling, power them, etc etc. Well, IMHO, you should have a bass made in heaven to make it worthwile.
My system provides me an incredible bass, deep, toneful, dynamic and with BIG slam (it shakes the room), when the recording has it. More it would be too much and not real.
Unless you hear only electronic, the heart of music is mostly on the mids and to have body (and good tone) in the music you need to have a well performed (a little boosted) frequency range between 200 and 400 Hz (said by experts like Jim Smith) and subs dont get that high. 
Also, musical subs are expensive (about 1k each). If you need two or more, plus the speakers @jazz99 has (Focal Aria), you will need a budget around 3.5 to 4k, with out cabling.
And for that money you can have a VERY nice pair of floorstanders with all the frequency range (for music) covered very well.
IMHO, for similar money, you can get much better sound with the floorstanders, spending considerably less time and effort. 
Just my 2 cents.

Plga, are you using the QC 500’s yet?  If so, you are not getting foundational bass response...you may be pleased with what you are hearing but 41hz @ -6db is not room shaking.  Stereophile even mentioned that a good sub would be beneficial.

Yes, Im using the Q Acoustics Concept 500 and I'm getting more bass shaking than I need.
In some reviews they mention that the bass is outstanding in this floorstanders and I couldnt agree more. It has everything, impact, dinamic, tone, etc. They have fantastic bass.
It is also true that I boosted the bass a little bit with Dirac, both the low and upper bass, along with the rest of the Digital Room Correction the sofware does.
Its incredible the bass you get with a nice pair of floorstanders and Room treatment (active and pasive).
I strongly believe you dont need more (for music) or, to have it, you have to spend a considerably amount of money, time and effort.
How much would it cost me to buy two subs at the level of my speakers??  at least 6 to 8k... and, how much time would I have to spend voicing them to the room? not even considering the space they need, that most of us don´t have. I should better spend that money on electronics as Im sure I would get much better results as I did.
Im sure with subs you can boost the bass more and have a some more impact and deepness, but for me it could sound unreal if you are not careful.
I repeat, IMHO, the musical experience is much more complex than just adding extremely low bass (under 40 Hz) and if you dont do it extremely right, Im sure you will mess up the sound of your speakers with the subs, because it happened to me and it was NOT subtle.
I repeat, if you want to get your system to sound as real music as possible, you have to address many different variables: standing waves, tone, soundstage, dinamics, detail, smooth highs, etc. If, and only if, you succeed to match the subs perfectly your speakers, IMHO, you will only get a part of the problem resolved.
And if you listen to music other than electronic (jazz, blues, rock, accoustic, etc) getting a deep bass wont give you much as many recordings in that kind of music DONT have it.
Again, just my opinion.

Glad your happy plga...better off not knowing sometimes anyway.  Saves money!

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.