Proper speakers should have high pass or low pass filters built in which are appropriate for that speaker's design.
You're missing the point: by the time it gets to the speakers it's too late. The whole point of having a subwoofer is to take the heavy-lifting off all the other speakers. In order to do that, the low-frequency content must be sent to the subwoofer in the first place
. That has to be done via a crossover at the driver level, because otherwise the low-frequency content goes to the individual channels, isn't on the CSub channel at all, and thus your subwoofer is essentially a useless lump, doing nothing.