A subwoofer is only supposed to play low, low frequencies. Preferably below 80 hz but that can vary somewhat and some can theoretically play a bit higher. Not too much though.
Likewise, a subwoofer out is only supposed to send low frequencies to the subwoofer. Ideally, 80 hz is pretty much a standard but, again, there can be some that go higher but not too much higher.
What, exactly are you shooting for? Most music doesn't really have a lot of content that really requires a subwoofer.