Originally Posted by Braunbear
The fundamental frequency alone does not a note make, in most cases.
There is not an instrument in the world whose overall voice is not defined at least as much by the harmonics as by the fundamental. This is, uh, fundamental. If your system plays down to 60 or 80 Hz, you can tell what's going on in the bass without the fundamental being present at all. This also is relatively fundamental. A violin would sound unmusical if you only listened to the part of the tones that go to the tweeter.
You can get most of what's going on in any music from your laptop speakers or an inexpensive old am radio. The midrange carries much of the meaning. But it sounds better, and IMO, you get the players' intent better if you have a tweeter so you can hear the treble above 2KHz or so. Similarly, depending on the program material, it sounds better with that last octave or two of deep frequencies.
But no, a crossover does not take all the bass instruments and put them in the sub. Sorry if you thought so. It just moves the deepest frequencies to the sub, leaving the harmonics in the main speakers' woofers and tweeters. Similarly, the crossover inside your left and right speakers puts the higher frequencies into the tweeter where they're reproduced better than in the woofer.