It looks like this car uses the same base driver set up as the Countryman. Someone out there must know the crossover frequency used between the speakers in the floor and the midrange in the door, but darned if I can find it by searching.
Is it the same frequency with the base and upgraded systems?

 

This is probably the best place to start looking for info about the MINI audio systems. I think it was written prior to the introduction of the Countryman series, but it's a start:

https://www.northamericanmotoring.com/forums/navigation-and-audio/216180-mini-audio-basics-specs-details-and-upgrades.html

 

Thank you. The answer is there, sort of. If I could read the graphs, I could figure it out, but the print is so small I can't make out the frequency axis. I tried blowing it up but the cheap program that came with my PC just made a large blurry image that I still couldn't make out.