Do you have a sealed or vented (ported) box? If you have aported, do you know what the tuning frequency of the box is?

Regarding the x-over point, I would suggest startin with about 100 Hz. Play the system at a medium level, and listen to it. Adjust the x-over frequency until it sounds best to you. Depending on the box and vehicle, you might go as low as 60 Hz for the sub. It also depends on how much mid-bass you have in the car also...

Let your ears be the final judge though....

If you have a vented box, I think Diamond suggests a 26 (or 28-ish) subsonic filter. I forget the exact number, but 28 and 32 are sticking in my head for some reason....

John