The recommended minimum crossover frequency of the Radian is 1.2 kHz, and imo this makes sense, as its impedance peak (and presumably its resonant frequency) seems to be about 850 Hz.
It sounds to me like your crossover doesn't include any equalization of the driver's response. If that is the case, then whichever driver has the best innate frequency response on the horn you're using will probably "win".
Ime frequency response is what matters most, then once the frequency response is good, the Beryllium diaphragm's freedom from breakup starts to be a worthwhile improvement. But Beryllium (and other exotic diaphragm materials) can't compensate for having an audibly worse frequency response.
I have some eq built into the design but it is probably not exactly right. Once I measure then I will know.