4.7uF is a tweeter value cap. 47uF would be getting closer to whats needed but theres an impedance interaction to consider from your SP12B's impedance peak and the capacitor. perhaps a series network between your K15 and the 12" would work ok.
a 2nd order network might be more practical - post some questions maybe at DIY Audio.
I suggest you get some means of measuring your speakers such as ARTA or even a spl meter and pc sin generator.
look what might happen in a typical situation with 4.7uF - 47-uF - 100uF with 4.7mH
-Black line is the speaker without any highpass
and again with a 10mH choke with 4 ohms dcr
