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

@plga - thanks for sharing your experience.  

I've often wondered if it's worth the trouble for me to pursue the bottom octave knowing that it will be difficult to get right.  Sounds like it can be done but be prepared for a daunting task.

For now, I'm going to keep it simple.  

@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.