It depends on what kind of equipment you have and what kind of input/output your subwoofer provides.
Take into consideration that while the included bass unit, while it doesn't go very low, covers most of the frequency range a typical subwoofer is supposed to cover.
If you really do want to add a subwoofer, you should set the LPF pretty low.
If you're trying to replace the bass unit with a subwoofer, consider that the amplifier and crossover filters are built into the "bass unit".
I have a polk subwoofer (powered) and an auzentech forte. I tried plugging the sub into the c/sw input on the soundcard with a 3.5mm to RCA cable. Not working. Ideas?
And yeah I'm not trying to replace the swan midwoofer.
Thanks!