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?

your neighbors won't like matter what you get...bass waves penetrate just about anything. That said I'm using a von Schweikert VS1 which you can pickup on Agon for well under $1000
Even if the subwoofer goes no lower than your main speakers, it will do it with less effort (distortion), and will relieve the main speakers of the task of LF reproduction, and that will make the main speakers better in the other part of the frequency range.
Go with that sort of sub. The lower frequencies are the ones that travel. If you had a sub that did 20Hz your neighbors would shoot you or get you evicted.
38Hz is a figure. The sub DOES reproduce notes below that, but at less volume. 38Hz is likely the 3Db down mark, a standard measurement.
Even with a sub that goes only to 38Hz you should be careful in placement, as the resonances could get ugly in a neighbors apt. I am in an apt in a 'over 55' building, so I cannot make ANY noise or be evicted. So I have my sub on a 2" thick concrete patio slab on rubber feet. It really keeps the vibration in my apt instead of my downstairs neighbors.
I like a small sub because it gives a normal bass sound at low volumes. So in a Jazz quartet, you get a true bass note from the bass. Nice.
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.
YES: Eldartford has an excellent and correct point concerning the improvements a sub can have on your system. That's providing the subwoofer system has a means of subtracting the freqencies it's reproducing from the mains(especially if it relieves the main amps as well). I've been actively bi-amping for years(pro and home) for those reasons, but never used an add on like the Velodynes. Do they incorporate a crossover to relieve the mains?
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.
I don't recommend sub if its not setup properly
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

i thought the rel, and some other subs, recommended running main speakers full range and simply running the sub high pass and using the on board sub xover to dial in the subs upper frequency cut off. for small, relatively well though out monitors, like my fostex 167e-based tektons, i want to get everything they reproduce and just augment, at the lower edge, since the main's aren't digging all that deep anyway, say 60hrz or so. anyway, that is what i do and am generally pleased, though my amp situation prevents me from comparing it to this other way of doing things.
In a small room at low levels (which you'll be listening at due to the neighbors) any small sub will perform well. I agree completely about high passing your main speakers. Most of the Velodyne subs I've looked at have a first order high pass filter so the mains will still be contributing quite a bit of bass.

You might also consider the SVS SB12-Plus
The Rel T1,T2 and T3 are very compact and may do the trick for you with some effort. The effort will be required to isolate the sub from the floor as they have downward firing woofers. I have a T2 and it works quite well with my Piega P2 LTD monitors. I do not have the same concerns as you though as I live in a single family home.
As for the hookup, there are three wires on one end of the Neutrik Connector. Two go to the left speaker terminals and the third goes to the positive right terminal. I suspect that your salesperson may not be too familiar with hooking up that particular type of sub cable ???
If at all possible, maybe your dealer can let you borrow a demo and this would enable you to get a better grasp of whether you may be able to isolate it from your neighbours ears and preserve your safety...LOL
Either way, the REL is definitely worth an audition.
I was never a fan of subs in two channel environment but I am certainly a believer now.
I hope this helps.
Thanks, BJ.
If he were to go with a down-firing woofer: he could always park it on a nice, flat slab of granite. Easy to find, and fairly non-resonant.
To help you isolate the sub from the floor use cheap "Subdude" platform from Auralex. About $60.