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

Depends on room, budget and goals really.  What is the current setup, wish list and desires for sonic bliss...musical tastes etc..

Yes, indeed. I agree Dave.

Just to mention, last night I've read the chapter "Stereo System Bass and Subwoofers" of the book "Get Better Sound" of Jim Smith, an interesting and educational book from a well known and respected expert on the audio field with several decades of experience. I didnt read this chapter before as I dont have subs in my system.

Well, Jim starts the chapter with this: "One of the greatest dissappointments for me has come from listening to stereo systems that incorporated subwoofers at shows and in home installations. I suppose it's the complexity of getting so many variables correctly adjusted".

Then, intrigued by it, I kept on reading the chapter so he provides many advices to install de subs (he suggest two for stereo) and smoothly blend them with stereo speakers. Well, it aint simple at all as I thought.

It also says something I found very interesting, as posted by @shadys telling his experience. For Jim, well installed subs can really improve soundstage holography. Ive never though it possible! I thought 3D sound came only from the tweeters, but it seems that long bass waves have somehow spatial information that contributes with imagin. Incredible!

Well, I would LOVE to hear a good pair of subs "well matched" in a nice high end system, to see how improves the sound of it. Im intrigued.

Nevertheless, do I recomend a sub for an entry level audiophile? Not, for sure. 

Again, just my opinion.

The human audible hearing range is 20Hz to 20,000Hz

No one on Audiogon I bet, lol

plga- 

I thought 3D sound came only from the tweeters, but it seems that long bass waves have somehow spatial information that contributes with imagin. Incredible!

While we don't hear low bass frequencies as localized sounds within a sound stage the way we do upper bass, midrange and treble, really low bass does contribute to imaging by imparting a sense of space. Really good low bass like you get with the Swarm or other distributed bass array can put you more fully in the recording venue. This reviewer explains it beautifully.  http://www.theabsolutesound.com/articles/audiokinesis-swarm-subwoofer-system/

There is more to what is going on here than just the perception of what is happening in the music itself. There is also a quite startling spatial effect. We are all accustomed to being able to control room effects in the mid and higher frequencies by acoustical treatment. But this is harder to do in the bass, and in most rooms there is a definite residue of spatial signature carried in the lower frequencies. The bass is one of the main ways you sense the size and shape of the room around you, and typically the signature of the recording venue is overlaid or even swamped by the bass signature of the listening room.

But the Swarm, presumably because of its smoothing out of response, largely suppresses the bass signature of your listening room. And as a result one can hear the original venue in a most unusual way. I played in a performance of the Sibelius Second Symphony recently, so I was particularly attuned to the acoustic effect this piece makes in a concert hall. Listening to Berglund’s recording for EMI with the Swarm in action, I heard a remarkable facsimile of the size and envelopment of the actual symphonic sound emerge. At the end, I felt like standing up and cheering, so close did I feel at some basic level to the real experience.

Yes. Even at $499 some of the suggestions here should provide increased depth and bass when properly positioned and setup.

Clearly the addition of a subwoofer/s can dramatically alter a systems presentation which might be described by some enthusiastic new sub owners as added clarity.

Keep in mind an investment in a higher quality subwoofer may service you for a lifetime despite all your other audio related upgrades. 

Good luck with it.