Lot's of good theories here, including Chadlinz's input. It depends, though. In a perfect acoustic space and setup, you could find the 3db down point of the mains, and ajust the sub to mate. However there are other variables.
A main variable is that typical receiver and passive speaker combo's opperate much more efficiently - regardless of response potential of the mains - crossed over to a powered subwoofer at a certain frequency range. THX finds that at about 80hz. Set up well, I find that a superb location, personally.
Basically, even if your mains can play down to, say, 45 hz, I know from experience that you can get my better dynamics, dynamic range and power handling if you cross em over at 80hz, and let the active sub woofer handle the dirty dubties. The receiver (being week on current, relative to separates amps) needs all the dynamic help it can! Also, the typical medium sensitivity passive speaker also needs much dynamic help, as it can't control the drivers half as efficiently as an active configuration.
Another consideration, as hinted at here above by others, is that the room is a huge factor. You might find, for instance, that the best location - all factors considered - for the speakers might be where they have a down point, or even hole at a certain frequency, where they'd function better crossed over elsewhere. So it depends. Same can be said about the subwoofer, which needs care in placement.
So, it's a balancing act, with lots of variables to consider. Basically, there's no "this is THE BEST setting, er whatever, across the board.
Hope this helps
