What about an active crossover, like the sony XEC505 ?
It has a four channel input (front L/R and rear L/R) and a six channel output (Front, Rear and Sub). You can change front, rear and sub -levels individually.
Does a great job in my car!
_________________________
---------------------------- MK1: 00314 (4GB) MK2a: 030103104 (30GB) Installed in a BMW 323ti