As I mentioned above, this can be a can of worms. Speakers are all about tradeoffs, and as you move your mid crossover point down, what are you trading off?
I also don't recommend crossing over that low. Baffle step will address the dip. Or crossover at 500Hz will cover most of it.
Designing for the BP102 is a puzzle, trying to make all the parts fit just right. As you go down in frequency, you begin to limit your choice of mid speakers. Eventually, it becomes another woofer, and you lose the higher end. Or full range, and you lose efficiency. Tradeoffs.
Just because a speaker says that it goes low doesn't mean that you should send everything in the frequency range. Some crossovers have a steep rolloff for a reason. It is best to be conservative in sending a lot of low frequency power to speakers to keep from blowing them.
You don't have to listen to me, but the BP102 is a common speaker and others have looked at it. You might do a search on the BP102 and see what someone like Bill Fitzmaurice says:
"If the box is too small to support this option I'd go with a single BP102 and an Alpha 8MR, with a crossover at 800 Hz. The sealed back mid makes an separate sub-enclosure unnecessary."
"The Alpha 8MR can work pretty well, you just need to design a good crossover for it that pads back the output, protects it from damaging low frequencies and integrates its response with the woofers."
Like Arjank, Bill suggests 800 Hz. And when you design the crossover, protect the mid from damaging low frequencies. Like 250 Hz.
