This little bookshelf started by looking at excellent fr response of Aurum Cantus 5" midbass AC130F1. Originally I wanted to pair it with small Dayton dome tweeter, but sound was lacking something. Fountek true ribbon in its horn loaded offering has excellent spec and sounds great. These two are meant to work together. I used rather simple crossover, first order series crossover, with just one cap and one inductor, at 7kHz. I selected this crossover frequency to suppress small breakup bump on midbass. Plus ribbons like to be crossed high. I added baffle step later, to compensate for rising response. Just one inductor in parallel with resistor.
Below are some pics from setup with mini dsp crossed at ~150 Hz to subwoofer. You can see how well the little speaker measures, almost ruler flat, this is in room response. First response is just the bookshelf alone, second one is with the sub. Off axis behaviour is exceptional.