As I understand the CS-2 is bi-amped (active design). One way to use them with passive speakers would be to use a AV preamp. The passive speakers would require their own amplification (either three mono-blocks or a 3 channel amp) and the CS-2's/other amps would plug straight into the preamp. Relative volume would have to be controlled via the preamp or volume controls on the mono-blocks/3 channel amp.
The alternative would be, as I think you're suggesting, would be to eliminate the existing amps/low voltage crossover, replace with a high voltage crossover, and plug straight into an AV receiver or multi-channel amp. To come up with such a crossover would require knowing the individual driver specifications and the knowledge of crossover design (certainly not me, sorry).