Crossover Design:

The active version of the B&C 15CXN88 crossover is based largely on the passive version that was developed first since I felt it I had it well dialed in.  Crossover point and acoustic slopes are essentially the same as is the overall response shape. There are a few addisional fine tune corrections on the active implementation like more precise notching of the HF breakup peak but the overall voicing is much the same.  The biggest difference is going to be bass extension, the passive design has a fullspace f3 of roughly 100Hz in the sealed cabinet it was designed in while the default active tuning extends this to ~55Hz giving a much more full and solid low end.

Even though this is an active crossover there are passive components used as well.  Essentially a 1st order filter on both drivers.  For the high frequency driver the capacitor acts as a form of low frequency protection as well as dropping the level in combination with a fixed resistive L-Pad which helps lower hiss on noisier amps/DSPs to acceptable levels.  The damping from the parallel resistance also prevents the HF diaphragm from resonating when the woofer is being driven, an issue I only realized was happing during well into the passive crossover development.

On the woofer side I am using a 1mH air core in series with the driver, I used a 15 gauge coil to keep DCR low but it could be an 18 gauge with only a very slight loss in output.  The increasing series impedance provided by the inductor as you move upward in frequency provides an obstruction to the back EMF voltages that are generated by the driver while being driven.  In some way it can be thought of as an external shorting ring.   The benefit is a reduction of odd order harmonics in the midrange of ~5dB when compared to  direct wiring the woofer to the amplifier.

Crossover sim with individual driver responses and reverse polarity notch: