Other than being active I struggle to see the connection between these three speakers.
IMHO - if you go ATC then the passive SCM20's or SCM19's are the way to go in the mid price range and then active for the three way models. The crossover is already pretty high on the driver used in the 19 and 20 and therefore, even active, you are substantially driving most of the important audio range (bass, mid-bass and midrange) with one amp - so the advantages of IMD reduction will be relatively modest. In the three-way you get to separate the entire midrange from the bass amplifier which is where there is a big advantage...just two cents as usual. (Other two way active speakers are generally crossed over much lower - so my comments would not apply to these)
IMHO - if you go ATC then the passive SCM20's or SCM19's are the way to go in the mid price range and then active for the three way models. The crossover is already pretty high on the driver used in the 19 and 20 and therefore, even active, you are substantially driving most of the important audio range (bass, mid-bass and midrange) with one amp - so the advantages of IMD reduction will be relatively modest. In the three-way you get to separate the entire midrange from the bass amplifier which is where there is a big advantage...just two cents as usual. (Other two way active speakers are generally crossed over much lower - so my comments would not apply to these)