NHT SB3, -3 db at 39 Hz in a sealed enclosure, so typical room gain should realistically push even a little lower than that.
And you believe that number? My Energy Veritas 2.3s claim 35 Hz at the -3 db point (it is more like -10 db at that frequency), the reality is more like 70 or 80 and they are floor standers with considerably more bass potential.
Here is its next bigger brother's real numbers:http://www.soundstagemagazine.com/measurements/energy_veritas_v24i/
Energy claims 29 Hertz at the -3 db level.
Now that speaker is a waste of money, why add a little more bass extension, wall interactions (it has a rear port) and an extra $1000 for something the subwoofer can do better?
Even if they did go that low, which I really don't want, it is probably a bad idea to use it, it draws a lot of extra power, and is usually accompanied by lots of distortion once you start getting any volume. I cut my speakers over at the THX spec using a THX crossover (although the slope isn't a THX slope because the speakers are full range, THX requires that they start falling off around 80). The bass is much tighter that way, especially the mid-bass.
You can also tune the bass around your room resonances by moving the sub around and putting absorbant materials where they will do some good. I have a plus 10 db room gain at 90 db at 60 Hz, so I tune the sub around it. You can't really do that if you run your speakers full range unless your preamp/controller is doing it.