this is the sub i have. its epically old but it still kicks hard enough running on 10 watts to feel at the other end of the house.
it has 2 inputs, as i said, left channel and right channel, and 2, corresponding i think, outputs. what you said is right, it passes the high frequencies to the outputs. this is the rear view of my amp:
(pic wont show for some reason, go to link if you want to see it)

as you can see, there is one output for speakers from the amp and the sub requires 2 sets of inputs. i just made this post to make it a bit clearer.