Nothing wrong with a subwoofer with a passive radiator if it is designed well.
The idea is to be someplace between a ported sub and a sealed sub.
You get a somewhat improved efficiency of a ported sub with the extension of a sealed sub, and avoid the peaks that many ported subs have.
they also tend to be easier to integrate.
in your case with the kicker the enclosure is too small for another active sub
try to increase amplifier power if you can if you want more bass.
The idea is to be someplace between a ported sub and a sealed sub.
You get a somewhat improved efficiency of a ported sub with the extension of a sealed sub, and avoid the peaks that many ported subs have.
they also tend to be easier to integrate.
in your case with the kicker the enclosure is too small for another active sub
try to increase amplifier power if you can if you want more bass.