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

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

Well
Ive used a sub with a pair of bookshelf speakers.
Then I change them for a nice pair of floorstanders and I couldnt make them work well with the sub as it only blurred and masked the sound of the floorstanders, A LOT. 
My opinion is that it is VERY difficult to get a the same frequency integration of a nice pair of floorstanders with bookshelf speakers and subs. 
Today I have a high end pair of floorstanders, with some room correction and Dirac Room Correction software and the bass is oustanding, incredible, and with out blurring or affecting the sound of mids and highs. 
If I were you, I would sell the bookshelf speakers and buy a nice pair of floorstanders. Otherwise you will get into something pretty hard to resolve and you will very likely spend more money and time on the process.
Then I would spend money upgrading the source and the power amp and then time voicing the system to the room.
Im sure it will be much more productive. 

Unless of course you have and immense room, wich its not most cases. 
Mine is 14' wide, 28' long and 9' tall. 
Trust me, dont waste your time and money dealing with subs, if its stereo music what you have your system for.
Movies with home theater is another story and hobby.