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

A subwoofer to reproduce and enhance the lowest frequencies will definitely add "presence" to your music and movie watching. Most natural sounds are in the low frequency range. Furthermore low frequencies are felt by the body as much as heard by the ears.  Most modern amps that are designed for surround sound will have a dedicated subwoofer output with the correct phasing and crossover otherwise many subwoofer designs offer built-in crossovers.  The most extreme important task when setting up a subwoofer is ensuring the phase of the subwoofer matches the phase of the main speakers.

millercarbon:
"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."

Hello millercarbon,

     I completely agree with you that audiokinesis keeps delivering audio gold by sharing his vast audio knowledge and experience with his thread posts.  I always learn a lot from his posts. 
     Now, your pop quiz question has me stumped:
     What one single letter best differentiates Duke’s highly informative post from all the rest? 

     I really have no idea but I'll take a stab just for fun.  

     Is it 'G'? 
As in g, I didn't know that.

Tim

Today’s post is brought to you by the letter "S".

The letter "S" has the Super Power of being able to turn singular into plural. But not always! Due to a design flaw it can only do this when used at the end of a word. Luddite Forces in the Audioverse exploit this weakness. In a typical mind-trick they say, "This is the subwoofer you are looking for." See what they did there? The original quote was, "These are not the DroidS you’re looking for." They dropped the "S"! Pulling you back, always back, to the singular.

It’s a trap!

Don’t go over to the Dark Side. Unleash the power of the letter "S". Use four subs.

Subwoofers will not add clarity to a system. They will add impact in the form of notes you can feel as well as sound stage volume.

I’m not sure of the logic behind this statement, but it is at least subtly wrong. In a small two-way system, the woofer covers both (sorta) bass and mid-range. The large excisions of the bass modulate the mid-range, since the mid-range is superimposed on a vary-ing, larger, bass excursion. This HAS to muddy and distort the mid-range. And in my listening, it does (or did, its been a while)

Beyond that, yes, it will add weight, impact and richen the timbre.
Now, all this depend son proper setup, and you need to remember that the subwoofer must be phase aligned with BOTH speakers, and that phase alignment is perfect only for one listening position. Do the math. Its worth spending some time to get the position right, and if the crossover may be varied, to find out which one integrates best with your main speakers.

I really like sub-woofer / satellite systems if all the above are observed.
Some of my fav’s from the way-back machine were Spica TC-50is and Sequerra Met-7s with a decent woofer such as an Audio Pro (about $1000, 30 years ago).
G

+1 on ML 800x for configurability
600x is $600, ported and might integrate better with ported mains

A sub MUST have 0 to 180 phase control, phase invert, 3 and 4th order slopes in addition to level and frequency.

The ML x series do and have the advantage of being adjustable from a smart phone. With the ARC microphone and Anthem Room Correction software, room faults can be somewhat mitigated.

IMO, you should roll the low end out of your mains. This effectively increases main amplifier power and makes integration with the sub easier.  Do NOT use the sub the sub internal XO. Depending on the room and sub, you may choose different frequencies. A single polypropylene, Teflon or polystyrene cap can suffice for a start. See http://www.ielogical.com/Audio/SubTerrBlues.php/

Learning to use something like REW [Room Eq Wizard http://www.roomeqwizard.com/] will pay huge dividends

Adequate low end room control is big, expensive and ugly. Judicious furniture placement and main/sub location can get you most of the way there. Just don't listen in a sparely furnished room with bare walls and floors.

I disagree that one can't hear phase in the bass. Having spent 1000s of hours in studios with time aligned monitors, getting the low end phase as good as it can be is essential. When it's not correct, kicks and snares are flabby, upright bass strolls and live orchestras have no focus.

A good sub properly integrated improves the listening experience out of all proportion to the frequency band.