Doing passive crossovers for subwoofers is stupid.
If you are doing high-level filtering, coming off the amplifier, the inductor and capacitor costs skyrocket. If you try and use iron core inductors to save money, then you have hysteresis distortion. Even if you use air core inductors, then you are wasting amplifier power, because the inductors will add resistance to the circuit.
If you are doing low-level filtering with op-amps, well that isn't really "passive", and the distortion/jitter/noise isn't going to be any better than a digital solution. If you have a high-end system, $600 for a miniDSP Flex with balanced IO is pocket change. The ADC/DAC easily clears 16 bit resolution, and subwoofer integration is trivial with the DSP capability. With the op-amp setup you don't have a fraction of the flexibility.
This is a review and detailed measurement of the Minidsp Flex audio signal processor. It was sent to me by the company and costs US $475 for unbalanced and $550 for balanced that I tested. Flex borrows the interface from SHD and puts it in a compact package. Seeing how I really liked that...
www.audiosciencereview.com