The Delta/APT system will probably work out pretty well for you.

You'll naturally have to work out the crossover, and that's where your work will be cutout for you. Wavelength is less than 4". Mechanical rolloffs of the two drivers are fairly close. The HF driver isn't made to withstand a lot of excursion. So I don't know if you'll want to go first-order on this one.

Narrowing DI of the midwoofer would match the horn between 2.0kHz and 3.0kHz. Going to 3.5kHz would probably be fine, and I'm not sure you'd want to come down any lower than that with the APT.

The thing that will probably bug you most about this system is the bass response. If you really only need 100Hz, then that's fine, it's a midrange system. But if you need to go lower, you'll want some help from room boundaries. Don't make the box smaller than 0.5ft3 and put it in a corner if you want anything below 150Hz.

Good luck with it. Let us know how it sounds if you build it.

Wayne