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?

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If your listening room is small, then it will not effectively hold a low frequency sine wave. You would need a medium to large room to get the lower octave that some subwoofers produce well. That being said, I do believe a small subwoofer integrated into a two-channel hifi system can work well in a small room. I think achieving around 40 hz in a small space is possible without loading up the room. Also, some of the better subs out there come with a room equalization microphone that helps get rid of the major room modes which might be an issue in a small room.

I recently added a JL Audio Fathom F113 to my two channel system which is in a room that is 40ft X 25 ft with 11ft ceilings. This sub transfomed my system in almost every way for the better.
Thanks for all the input.

My local dealer has suggested a REL sub over a Velodyne. He said the Neutrik connection of the REL is preferred. He tried to explain about connecting it to the left speaker post on the amp. Am I crazy or does this seem wrong? What if a song has the bass mixed to the right channel? I asked the salesman that and he was flummoxed.
I'd consider a Buttkicker shaker and amp installed in the couch or listening chair, using the isolation feet the sell as well. The cost is small, the results impressive, and the neighbor disturbance factor zero. I have two Velodyne 15-inch subs PLUS the Buttkicker, and the combination is hypnotizing once you take the time to the levels and crossovers adjusted.
I think that Eldartford's explanation is one of the more important reasons to get a powered sub-woofer, it makes the main speakers perform much better; less IM distortion, and less stress on the amp.