With the crossover arrangement you describe, here's what would happen (assuming the crossovers to be absolute at the set frequency, although in reality they are a slope that operates around that frequency):

• Data below 40Hz from the left and right channels is redirected to the subwoofer.
• Data below 80Hz from the center and surrounds is redirected to the subwoofer.
• All "LFE" data (the ".1" channel in Dolby Digital 5.1 and DTS soundtracks) is sent directly to the subwoofer, where it merges with low frequency data redirected to the subwoofer from the other channels.

Does that help any?