@mahlman, that's correct. And I did stick them directly to the horn, which accommodates 4 bolts but with the same spacing. The active/passive/eq question will require some doing; I'll try to condense.
I do my active digital xovers with huge FIR filters (2M FFTs each) on an HP Prodesk i5 4-core (got it for $170 refurbished) and use a cm8828-based 7.1 soundcard ($50) for signal I/O. 7.1 is perfect for 3-way stereo with a sub. Despite the low cost they scope out beautifully. I make the filters on RePhase and run them on Convolver VST (which also does my delays and mapping). I use brickwalls with an EQ curve superimposed, all constant Q type and linear-phase. It runs through the foobar2000 DSP stack. All my VSTs are wrapped in Mux Modular, an excellent and stable VST wrapper (itself a VST). It's the only piece of s/w I paid for ($60, a bargain). Excellent graphic connection mapper with a ton of onboard processors, DAW stuff. I use before/after Sox resamplers to do the filter processing at 768 ksamples/s. The sound cards are limited to 192ks/s but they sound and test great. A similar computer is used as a front end to carry (or receive) my source material, also through foobar2000 with whatever processing I find effective, and sent through another 8828-based PCIe soundcard (in spdif stereo) to the crossover machine. There is quite a distance between them, hence the need for 2 machines. At these costs, so what?
The former CD horn I mentioned is an Eminence APT-150s. I used an adapter to mount the ring radiators. It really does have roughly 100x40 dispersion, and the ultra-high frequency dispersion is incredible. Tone bursts at 10khz are remarkably clear. But the dispersion pattern just didn't match my Atlas mids well.