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

I am convinced that adding a pair of subs 
1. Improves the bass range
2. Cleans up the midrange 
3. Widens the sound stage .
the key being placement and tuning ie the phase setting and volume of subs.
i use my pair of kef blade 2s along with a pair of Jl Audio f113 v2. The sound improvements after adding the subs were phenomenal through the entire spectrum.
i am not a profound supporter of room sound correction programs .
noble100 was right on the dot till recommendations no 5, from that point on I tend to differ from him. Not saying he is wrong but saying his techniques are different .
i would recommend you contact Barry Oder ie the sound doctor. His experience with subs spans over 4 decades. I spent a weekend with him and learnt a lot which I have used for setting up my sound . If you call Jl Audio for help they would guide you to the white papers written by Barry . He is a true guru .
http://www.soundoctor.com/whitepapers/subs.htm
Read the above paper written by him and you can contact him over phone . He is a tremendous resource and loves to help people who love bass. Who doesn’t . 
You don’t need a Jl Audio sub to make great music but you need to have it perfectly aligned with your main speakers to sound great .time domain is in my opinion crucial . 

Only way to know is to add a sub. I’ve always found it to be complimentary to any system as a sub can make movies really go wow! Not sure about room correction, I move the sub around and tune it constantly until I achieve what I need to integrate it. Sure there will be imperfect moments within certain songs but overall it’s much more fun to listen with a sub.

I would add a pair of REL T/9i's.  I listened to the Focal Aria line and I kept asking myself where is the bass.  I also asked myself if audiophiles do not like bass in general.  Even their largest 948 aria speaker lacked in bass extension.  Adding two T/9i's will really help.  

The best thing about subs is that you don't need them when your speakers are good. And if they are not good - replace them. And don't forget good source, electronics and cables.

More Audiokinesis gold:

The ear simply does not have the time-domain resolution to hear a slight timing mis-match in the subwoofer region. The ear is UNABLE to even detect the presence of bass energy from less than one full wavelength, compared to which small timing errors are insignificant. What the ear IS very good at, is detecting SPL differences in the bass region. This is implied by the bunching up of equal-loudness curves south of 100 Hz. So the ear can indeed hear something going on as you fine-tune the phase control, but what it’s hearing is the effect on the frequency response.

If the main speakers are a bit muddy in the upper bass, you can have the subs overlap them a bit and then use the phase control to dial in some cancellation, thus improving the clarity.

I’ve had many customers claim to hear an improvement in clarity from the addition of subs without highpass filtering the mains, and I’ve heard it too, but cannot explain it, apart from the occasional situation described in the preceding paragraph. Of course there is also the improvement in clarity that can come from highpass-filtering the main speakers to reduce cone excursion, in which case the transparency of the highpass filter can be a factor.

Regarding adding a sense of depth or sense of envelopment/immersion, if you have two subs, you can place one at either side of the listening position and dial in 90 degrees of phase difference. This will tend to synthesize hall ambience. Credit to David Griesinger for this idea. The 90 degree phase difference ("phase quadrature") will also tend to improve the modal smoothing.

Sometimes adding a subwoofer to a tonally well-balanced system tips the spectral balance to the dark side. When that happens, we can bring balance to the force. We can add a rear-firing tweeter to just nudge the spectral balance back to normal, without messing up the imaging by adding another source of first-arrival sound.

Duke

More pure gold. I’ve copied the whole post just to pose a pop quiz and see if anyone can answer: what one single letter best differentiates Duke’s highly informative post from all the rest?

Hint: to understand the answer is to understand the problem.