The reason some speakers go beyond 20KHz is simply the tweeters are so good, which also implies to the frequencies we can hear. Also some manufactures claim there are sound advantages too
Good subwoofers are usually only designed to go up to 120Hz, in-fact the lower the sub can go the worse it will sound around the 100Hz region.
e.g.
A cheap sub that has frequency range of 40Hz to 150Hz will probably do a better job than small speakers at anything under 120Hz, even if the speakers can do 70Hz+.
A good sub that has frequency range of 20Hz to 120Hz will usually struggle with frequencies above 80Hz, but then you need good speakers that can handle 80Hz with ease.
My setup is a compromise, my R270s and R225 are spec-ed to go down to 50Hz and 55Hz, so they handle 80Hz with ease and also sound better at 80Hz, but my KEF rears are spec-ed to 70Hz, but in reality they sound best at 120Hz, but as they are only rears and they can go to 80Hz (actually recommended by KEF) my crossover is 80Hz.
If your speakers are spec-ed at 100Hz then 120Hz will probably be your best bet, I would even explore a higher crossover, do what ever sounds best to you.
The source of low frequencies cannot be placed - which is why you only need a single subwoofer. The higher the frequency, the more easily you can determine its source. 250Hz is high enough that you can easily determine the source and is therefore too high. 80Hz is around the border and is therefore the THX crossover frequency, but with satellites / bookshelves, 120Hz or even 150Hz is more common.
You should get hold of a SPL meter from you local electronics shop and a low frequency test CD (download here). Using these you should configure your receiver's crossover and your subwoofer as per its instruction manual to provide a smooth crossover at the lowest frequency >= 80Hz that you can reasonably obtain without leaving a hole (see above test CD web site). There will be some overlap where both the subwoofer and the speakers start to drop; it's the sum that you hear and need to smoothen.
Many tweeters have no problems producing frequencies about 20kHz. Normally manufacturers don't bother telling you about this as it's completely irrelevant (unless you're a dog) and CDs have an absolute hard upper limit of 22.05 kHz (half the sampling frequency). With all the number games going on, some manufacturers have started to state the actual limits of their speakers, but figures beyond your hearing remain meaningless. 20kHz, BTW, is generally only reached by a few lucky children and young females.
