It is a gradual mixing at the crossover point. It's not like the speaker cuts out at 80Hz and then the sub plays, but more like an overlap to help blend the sound.

Also, the higher the crossover point, the more you're ears are able to locate the sub. Not necessarily a good thing. Play with the crossover and listen to the same song over and over and see how the sound changes.

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"Nothin' up my sleeve. . ." --Bullwinkle J. Moose