Crossover for Apogee Stages + Audiom 13WX

[Quantum Waves]

This is my first post on DIY. I am certainly no expert speaker builder but I have learned a lot this year. I recently finished builing Tony Gee's Andromeda Speakers and they sound great. I also really like the sound of my Apogee Stage ribbon speakers so I got the crazy idea to combine the Focal Audiom 13wx woofer in my Andromeda's with my Apogee stages and was blown away with how amazing they sounded. The Stages are being driven by a pass labs x-150 amp (match made in heaven) and the 13 inch focals are driven by an old 100 watt adcom amp and pre-amp where I can adjust the volume of the big woofers. I'm using the same crossover I had in the Andromeda speakers which I'm sure is a big no no since there is not midrange or tweeter connected to them. Apogee made a Mini Grand that used two 8 inch sub woofers and an outboard DAX crossover network. I would like to do the same thing by designing my own crossover. The DAX had its crossover frequency range at 24 Hz to 80 Hz with a rapid fall off in the stop band and designed to match the Apogee Stage's low 3 ohm impedance. The DAX crossover slope is 6dB gradually increasing to 12 dB.

Does anyone have advice or experience on how I would go about designing this crossover? Or has anyone had experience adding subwoofers to their Apogee Stages?

Thanks in advance.