Well if you're getting the full component set with the CD1E's, the external passive crossover comes with it, but if you are getting JUST the horns, as I have seen for sale sometimes on Ebay, you would either have to higpass them andpower them with their own amp like other horns or buy that crossover. The crossover is a passive crossover like one that would come with a traditional component set, except that it has some equalization settings for the horns. So if you got the full set, it would come with the horns, crossover, and mid/midbass. The only highpass you would need would be between the woofers and your subs and you would only need 2 channels of amp power to drive the full componet set (again, like a traditional component set with passive crossovers). For the record, I have not heard these, but certainly love my CD1Pro's! Next car, in maybe 2-3 years, I'll step up to the CD2's or the newer models that will probably be out by then.

Actually the CSX pasive does nothing as far as crossover functions for the Cd1E horns. all the CSX passive has some attenuation switches and also inline resistors to give the nominal impedence to theamp, based on what mid is used.

the cd1e have a natural roll off around 1.2khz.
You dnot need to externally highpass them. they really have no real impedence, its is in the miliohms, b/c they use a lightbulb as a resistor.