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

@vtvmtodvm --

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The "distributed array" concept is worthy with one notable exception: It generally (?) recommends leaving the main speakers UNfiltered. While that can be beneficial with respect to reducing low frequency room mode response, it ignores the primary benefit implicit when using an active electronic crossover to separate the ultra-low (<100Hz) bass passband (subwoofers) from the low-&-mid bass passband (main speakers). That separation assures that the main speakers (and their power amplifier) no longer have to process any heavy low bass energy. Those low bass frequencies now get shunted to the self-powered subs. That’s an important advantage, and it’s way too vital to bypass.

Glad to see this pointed out, because it’s an important benefit high-passing the mains as suggested. To quote:

Some audiophiles don’t want to introduce yet another active "thing" in their precious signal path, not realizing that adding the crossover is very much the lesser of two evils.

Actually adding a crossover is really a WIN-WIN situation:

WIN # 1) Since you are now NOT putting in 20 Hz - 80 Hz into the mains you are not using up the available LF cone movement with bass, so the LF cone in your mains is able to play its higher freqs (up to IT’S crossover point) much more cleanly. You get an apparent 6dB or more dynamic range. You can play your system LOUDER, and also with less compression distortion in the LF driver when you’re having that Saturday night dance party and you’re playing urban bass technopop at 110+ dB. Really.

WIN # 2) Since you are not putting bass into that same driver you are not Doppler modulating everything between 80 and 600, or whatever the next crossover point is. This means cleaner mids. By far.

WIN #3) You are not sucking current out of your main power amp at low frequencies, so there is more current reserve to play those highs louder...

WIN # 4) Since the cones aren’t moving as far at the low freqs the driver itself is not generating as much back EMF therefore the damping factor and all of its issues are greatly negated. And you don’t need to run silver plated cold water pipes to your mains as speaker wires because there is less current draw by the speakers.

WIN # 5) Freqs below 80 are now NOT causing transient intermodulation distortion with the higher freqs (and vice versa) in your power amp. Cleaner still.

http://www.soundoctor.com/whitepapers/subs.htm

You also, with your earlier post, pointed to the importance of the distinction between phase and timing, the latter aspect being my main issue with the multiple sub arrangement, which has coverage as its main goal (and not uniform time-of-arrivals). I’ll vouch for the importance of using only a limited degree, if any PEQ while having fairly uniform coverage via the much lauded, even preached-about-as-the-gospel mono-coupled 4-sub approach, but I find placing dual subs symmetrically to the mains, in true stereo, can hold a distinct sonic advantage, and this doesn’t rule out finding a placement that takes into consideration successful coverage with zero to limited PEQ; it may take more effort, but it’s (added) time I’ll gladly invest for the summed-advantage outcome.

For some reason stereo bass appears to be a controversial issue, with many claiming quite rigorously that the directionality of frequencies below some 80Hz goes out the window, and while I find there’s some merit to this (certainly insofar audibility goes) there are phasing differences nonetheless (when stereo information in the bass is found in the source material) that are clearly felt when running dual subs in stereo. This can come forth as a deliberate effect via especially electronica/techno music, and as an "illumination" of ambience in particular with classical music. Being audiophiles, many of whom I gather listens to classical music as well, why would you not take advantage of this?

I've chosen a 7th order (42dB/octave) Linkwitz-Riley low-/high-pass at 80Hz for my tapped horn subs and all-horn mains via a Xilica XP-3060 digital XO - it's simply what sounds the best. 

Previously I ran my mains full-range with a single SVS SB16-Ultra low-passed at 61Hz 24dB/octave (via its own build-in DSP), and while this pairing integrated very well my new setup with two tapped horns (powered by a Crown Macro-Tech 1200) and managed by the Xilica unit, now high-passing the mains, even at this rather early optimization stage sans PEQ offers better, indeed seamless integration with the mains, while lending the presentation a sense of uninhibited "breath," ease and coherency not heard with the previous single SVS-installation. 

Aspects of this no doubt falls back on the inclusion of not one but two tapped horns, but running the mains full-range, by-passing the Xilica, with a lower ~60Hz low-pass of the TH's tells me all I need to know with regard to my preference; to my surprise even running the Xilica over the mains with a high-pass isn't detrimental in regards to transparency, but has a freeing effect on the sound with noticeably better energy-coherence and overall integration. If there is something robbed of nuance via the Xilica, I'm not hearing it (in indication of how good a digital XO it really is), masked perhaps (perhaps not) by the other qualities it brings to the table with the high-passing of the mains that only adds to the enjoyment and insight of the sound. 

In not trying to make a case with the above as a generalization of what works the best in every situation (certainly, it depends), but simply to highlight that the typically non-audiophile approach of steep filtered, high-passing of the mains in the 80-100Hz region can work extremely well - with a pair (not four) of symmetrically placed tapped horns, no less, and in stereo mode, God forbid.