There's no easy answer to that one, just a variety of compromises. Basically, the filter should be wherever it needs to be to achieve your desired goals with the drivers you will be using and enclosure they will be used on / in. There is no fixed point; assuming you want to make a proper job of it, you work with what you're using, unless you're happy sticking a cap + coil on it & hoping for the best. The telephone bandwidth (midrange) as defined by Bell Labs 80-odd years ago is 200Hz - 4KHz. Some prefer not to cross in this region. The major power-demands for orchestral music occur < ~500Hz, so that is also a popular frequency. Neither account for diffraction effects etc., so some consideration needs to be given to that also, along with acoustic slopes &c.
So would you say something around 200 Hz is in the range of optimization. I plan on porting the enclosure for the woofer.
I am currently working my way through drivers. I will post soon. ThanksNo. Because I don't know what your design goals are, what the drivers are, and what the enclosure is (along with its dimensions). That was the point I was making in my post: there is no optimal per se, just a variety of compromises. You select what is likely to work best for what you are trying to achieve. Without knowing any of those, we can't really offer much in the way of advice.