Not all that convinced that FIR is inherently better than IIR, espescially at lower frequencies, but mostly due to pre/post ringing. More interested by digital Bessel filters.
If you just want a digital I/O xover, look at Xilica, which is one of the best in pro gear but no automated correction.
If you're going from the ground up with a speaker, I think this project with the Digmoda 552 is intriguing although I would do a few things differently.
http://www.audiocircle.com/index.php?topic=85662.0
Had similar idea a while back and considered a DIY modular DAC/pre with dual Wolfram DACs and BB PGA2213 chip attenuators on plug-in boards. Rejected series relay attenuators for the expense. Possibly even a 4X3 touch screen LCD or smart phone/bluetooth interface. No way would I have the time for a project like that.