Jin,
To amplify (sorry :-) what Greg said, the sub is inserted between your pre and your speakers by either using line level (RCA) inputs/outputs on the sub (inserted between pre and amplifier) or speaker level inputs/outputs (inserted between amplifier and speakers). Most subs have more flexibility in adjusting the crossover frequency (what goes to your sub versus what goes to your speakers) using the line level I/O.
To amplify (sorry :-) what Greg said, the sub is inserted between your pre and your speakers by either using line level (RCA) inputs/outputs on the sub (inserted between pre and amplifier) or speaker level inputs/outputs (inserted between amplifier and speakers). Most subs have more flexibility in adjusting the crossover frequency (what goes to your sub versus what goes to your speakers) using the line level I/O.