Nick,
I forgot to add that if you have the windows up, you can get amazing subwoofer performance in a car interior and for relatively little money compared to home subwoofers.
There are no standing wave problems because the sub pressurizes the entire interior, so any passenger hears the same powerful bass that you do. Subs for cars are all 4 ohms, so you can get more power efficiently from the amplifier. Sealed-box subs are inefficient so you want lots of real watts to drive them, but you get very smooth, tight deep bass to below 20 Hz from a well-designed sealed box car sub.
Alan
