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

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. 

@millercarbon: You have referenced Floyd Toole previously. If you look on page 154 of his book, "Sound Reproductions", you will see a chart of frequency by sound attribute (direction & space + timbre & sound quality).

100Hz looks like the lowest frequency for boundary effects and envelope.

My understanding of spatial cues is that it has more to do with the time domain (reflected sound) than the frequency domain. The pro audio guys on Gearslutz are maniacal about getting their ETC curves and sound models correct. (Good resource for this stuff.)

I think we all agree that deep bass improves the SQ.

A 0-180 degree control is NOT adequate.

It is with a Phase invert