I've made some progress
I've ended up using Jriver on a Mac laptop (it is a really badly written port of the PC version and not very stable) running through a Focusrite Scarlett 2i4, Mastersound EL34 amp for top and a Musical Fidelity A100 for bass.
I've settled on a 200hz crossover point for now with 12dB per octave slopes.
It has more bass than I thought it would, I'd always imagined that I'd need 2 15s per side in an OB. There are some nice things happening in the mid range and it seems integrated, you don't feel there are two speakers per side. Top end extends as far as I think you need although I will experiment with that at some point perhaps.
the bad parts are the mid range can get a bit harsh and shouty at high volume, it doesn't play particularly loud, but that might just be that I'm sitting so far away from the speakers (20ft maybe), the bass while loud on its own can disappear when other instruments come in and swamp it. This I think could be too high a crossover point or maybe just a function of the room not being pressurised like a large box speaker would and maybe this is more natural.
Will play around for a bit but I'm thinking some sort of horn mid experiment is coming.