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

Years ago I listened to a pair of magnepans at a dealer and the subwoofer was located a few feet behind the listening spot.  It was the first time I’ve experienced expanded height to a soundstage. When they turned the sub off, the stage seemed to collapse back down.  I don’t hear a lot of people with this type of placement as it usually seems to be more toward the front of the room near the mains.

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..