I have a computer model of the crossover that I manipulate that uses the actual polar measurements to predict the total field response. I adjust the component values to get the best overall response - the flatest response in the sound field from +30 to -30° across the entire audio band. The resulting filter is highly unlikely to be a "Bessel" or "Butterworth" or any other name brand filter. They are both 3rd order and are definately not tuned to the same frequency.