Presumably you're talking about crossovers at speaker level?, so you're talking VERY expensive, and you will lose a considerable amount of power as well. Assuming this is a stereo system?, and a mono sub-woofer, you also need two of them, and a special twin voice coil sub-woofer. A passive filter will also seriously reduce the damping factor, and the more complicated the filter, the more all this factors affect it.
There are good sound reasons why it's normal to use electronic crossovers and seperate amps!.