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

Hey Dave. 
Take it easy.
Im only telling my experience. I dont doubt about yours as you mention you seem to have much with very popular brands, but that doesnt make mine not valid. 
Im also aware that my room dimensiones are difficult as posted by millercarbon (thanks for pointing that out), but I cannot change them. As I said YMMV. 
I also have to say that I would LOVE to hear a high end system improved by subs as I cannot imagine it with out hearing it. 
Anyway, I still think that getting the best system your budget can afford and voicing it to the room should be the first step and then, with that experience on your back, if you are not happy or totally satisfied with the results, you should get with subs. 

All good!  Experimenting is fun...enjoy and happy listening.

Hey Dave
A couple of questions for you as you seem to have large experience with high end hear and knowledge in audio. 

As I understand, you and some other members in this thread, with more experience than me I guess, have managed to successfully match subs with high end systems with great results. 

1. Was it somehow simple and cheap to get it done? I would appreciate an honest answer. 

2. Would you really recomend, to an entry level audiophile (with little experience on audio), as I was just a couple of years ago and I guess (please no offense) Jazz99 is, the starter of this thread, to start improving the sound of his system by getting subs? Or would you recomend him, under your experience, to begin with something else first, like upgrading speakers, electronic and voicing them? 
Regards. 

Depends on room, budget and goals really.  What is the current setup, wish list and desires for sonic bliss...musical tastes etc..

Yes, indeed. I agree Dave.

Just to mention, last night I've read the chapter "Stereo System Bass and Subwoofers" of the book "Get Better Sound" of Jim Smith, an interesting and educational book from a well known and respected expert on the audio field with several decades of experience. I didnt read this chapter before as I dont have subs in my system.

Well, Jim starts the chapter with this: "One of the greatest dissappointments for me has come from listening to stereo systems that incorporated subwoofers at shows and in home installations. I suppose it's the complexity of getting so many variables correctly adjusted".

Then, intrigued by it, I kept on reading the chapter so he provides many advices to install de subs (he suggest two for stereo) and smoothly blend them with stereo speakers. Well, it aint simple at all as I thought.

It also says something I found very interesting, as posted by @shadys telling his experience. For Jim, well installed subs can really improve soundstage holography. Ive never though it possible! 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!

Well, I would LOVE to hear a good pair of subs "well matched" in a nice high end system, to see how improves the sound of it. Im intrigued.

Nevertheless, do I recomend a sub for an entry level audiophile? Not, for sure. 

Again, just my opinion.