The Sub channel setting should be set to low pass and set no higher than 80Hz.
This will, as SVT Colin put it, help to disguise the sub's location in the trunk
The Rear channel should be set to high pass and be set to the same as the Sub channel frequency.
This will keep from having any gaps in the frequency range as well as keep the music and bass inside the car.
The Front channel should also set to high pass, ans set a little bit higher than the rear channel.
This will bring the music to the front of the car.
