I've had my HT setup pretty much the way I like it and I usually don't tweak with it. But I was wondering about cross-over thresholds. If the crossover in a typical receiver is set to hand the output below 80 Hz. to the sub, then how is this cut-off really made?

Is it a sharp cut-off; anything below 80 goes to the sub and everything above to the speakers? or is it blended slowly, where it begins to drop off all the speakers at 85 Hz.-ish, the sub begins to pick it up there, and doesn't get everything until the 75 Hz.-ish point?