I'm going sealed because I don't need SPL, I need SQ. Thus the idea for separation of frequency ranges for less chance of distortion.

And as for the 4-way system, I might have a 12" in the trunk for 22-80Hz and two 8" for stereo 80-somethingHz somewhere in the cabin (me and I800 had a little chat in le chatbox). But where in the cabin to put them?....