Your car system plan looks really nice, although it's a bit complicated. Your top end design sounds pretty good, but the seperate woofer and subwoofer are unnecessary.

Any of the subwoofers you mentioned will perform excellent in a vehicle. I have heard of the Leviathan being used in car with amazing SQ results, the brahma is a beautiful sound quality woofer whichw ill run in tight cabinets, and can play loud as well. There are also some other good woofers out there for sound quality, like Crystal CMP's (I own a 15" model), Ground Zero subs, and Eclipse (although pricey). You don't need to use the bass cabinet at all. Any of these woofers will authoritatively handle 20-80 Hz in a car, where you can xover to the Audax woofers. I wouldn't run your fronts any higher than an 80 Hz xover, because the higher you go, you will actually notice a shift of midbass and bass frequncies to the rear of the car, which doesnt' sound good.

You can try porting all of these woofers (except the CMP, doens't sound as good) and you may or may not like it. Ported boxes in cars tend to have a rising response in the lower octave, which some people don't like. If you really hate that and don't have a good EQ to compensate, a sealed box would be a better bet.

My planned system uses a pair of Image Dynamics horns up front running down to 1.2 kHz, crossing over to four Seas Excel 7" midbass drivers in tiny ported enclosures, and finally crossing over at 60 Hz with an Eclipse Plutonium 18" woofer in a compact passive radiator box. The sub is currently in the prototype building stage and won't be available on the market for a few more months I believe.

The whole system will be running off an array of USAmps, with the horns and midrange drivers running off a hybrid tube amplifier. Fully digital equaliser and seperate active crossover, with a Nakamichi Cd-45z cd player as the "brain" of the entire operation.

Adam