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

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.  

In my humble estimation, a subwoofer adds those bottom resonances and harmonics that otherwise would be missing. Any subwoofer should not be noticeable as being a source of the bass, meaning that it is subtle and definitely felt and heard, but not pronounced as a sound source.

A good subwoofer will support all the other frequencies . . . and they, in turn, will do the same to the lowest bass notes, which can be attested in a balanced system where the subwoofer is turned off . . . and then back on -- the difference is most palpable and fulfilling in its supplementing the sound of the main speakers.

A good example would be a good grand piano. The soundboard resonant the harmonics as do the strings and the cabinetry. What you don't want from you subwoofers are the standing waves that excite the room into a boominess, and a sort of one-note wonder that emphasize certain frequencies in the deep bass. The subwoofer should be capable of singing and playing a bass melody . . . not just a stangnent rumble or irritating thunder for the sake of room shaking bass. To me, it is the qualty of the bass, the solidarity of the sound that give a very satisfying feel and completeness to the overall presentation of the music.

Yes...not that difficult folks.  REL gives a very musical result.  Move it around till you hear best bass integration.  Try even with plane of speaker mid/bass drivers facing parallel or horizontal.  

russmaleartist,

     Exactly, your description of what a good sub or subs integration should do and sound like, and not sound like,  is just as my expectations.   On my bass system, the subs are completely silent as long as there 's no music or ht content that calls for any reproduction of frequencies at or below 40Hz. They're like an elite group of very stealthy and extremely capable bass soldiers that never give away their presence but are constantly alert, awaiting the agreed upon signals being sent before instantly springing into coordinated action,  precisely performing actions with the proper application of force necessary to accomplish the mission, before returning to stealthy but alert silence once again and awaiting their next call to action. How's that for hyperbolic but accurate fun?  
     I believe a good sub system is not meant to be constantly heard but just when the content calls for it.  In my experience, attaining this quality requires  playing a wide variety of content and fine tuning the crossover frequency to be as low as possible but not low enough to prevent their contribution when needed. I've come to the conclusion that it's really just a balancing act with the best hoped for outcome actually being more of an approximation.  But it's also true that a good cutoff frequency setting approximation will be perceived as seamless integration with my main speakers 90% of the time.
       Conversely, setting the crossover frequency too high on a sub or sub system, imo, could be the most common and detrimental mistake many individuals make with their systems.  This only results in the subs being constantly heard and a perception of the bass being disjointed and not well integrated with the main speakers.  
      What particular crossover frequency is considered 'too high' is subjective but I think most people will know it when they hear it.  
I'll just state that, if you're constantly aware of a sub's or sub system's contribution, the crossover is definitely set too high.


Tim