most alpine amps(sub amps excluded) have selectable hi or lo pass x-overs(and adjustable) for front and rear but i believe you are using all 4 channels now,right?

my neighbor had an aftermarket pioneer head unit that was totally configurable with selectable high pass for front and rear,assuming a sub was being run,now the cool(and strange) thing was the hi pass only affected the speaker level out!
all preamp outputs were automatically high passed from where the sub was lo passed.hard to put into words but read this several times to understand what i'm saying.

if that is the case with your pioneer then you could easily drive the kickers with the head unit(assuming it has an internal amp)