Subwoofer: How low should I go?

My B&W 683s are rated at 38Hz - 22kHz.

I've been toying with the idea of a small subwoofer to augment the low end a bit. My apartment is pretty tiny, and in a crowded NYC building so I don't have room for a big sub and don't need or want earth shaking bass, just a little more oomph.

The sub I've been looking at is the Velodyne Microvee. It fits the bill size wise. But it's rated at 38–120 Hz.

My question is, do those numbers mean the sub covers exactly the same low end as my speakers, making it useless? Do I need to go with a lower frequency sub to hear a difference?

Good point, if they don't relieve the mains, the benefit goes out the window. This approach to improving the sounds the mains worked in an obvious manner when I owned Vandersteen 3As and bought a pair of his power subs - the midrange improved in a very noticeable and appealing way.
Is this to say that anyone who is merely tacking on a sub to an existing 2 channel setup is doing their system a disservice? I can only imagine the number of people who have simply plugged a sub in.
They are not gettign the full benefit of using a subwoofer; it still might make for a system that sounds better to the listener, especially if the mains are truly lacking below 50-60Hz (small monitors).
It is all about the quality.
What you do not want is a boom box.
Get Rel. Good value and good performer.
Size is also NYC frendly. I should know - I live here.

Happy listening and
enjoy the music