Interesting because what I have seen in most coax drivers without a dedicated horn is quite a bit more ripple in the response of the tweeter.VERY INTERESTING!!
Okay, part of what's happening MIGHT be this: The relatively high resonant frequency of the Radian coaxial helps minimize cone excursions from signals well below the enclosure's tuning frequency, which in turn helps to preserve the inherent clarity of the coaxial compression driver.
An alternative coaxial format is to use a separate horn for the compression driver. I have yet to find one with this format which did not have a nasty dip in the frequency response in the upper midrange region from the cone's output reflecting off the back of the horn. And imo it's not so much the resulting measurable frequency response dip which degrades the sound, but the delayed arrival of that distorted reflection. So I prefer the format Radian uses, and they do it very well.