As far as I know, most high end crossovers are designed and tweaked by the speaker designer. That's the secret. There wouldn't be an art to it if it was just making boxes and plugging in drivers. I've heard that many speaker designers spend many hours listening just to pick a better capacitor or inductor for their crossover board. Franco Serblin of Sonus Faber is just one of the pickier designers. Tweaking the crossover is what they also call 'voicing' a speaker. Just like tube rolling in a tube amp/preamp. Selecting different components can change the sound dramatically.
Regards,
John
Regards,
John