not sure what L box is but
in general words there are two kind of boxes commonly used
sealed and
ported
sealed boxes does not allow any air to move in or out of the box hence increaseing and decreasing the air pressure inside during movment of cone, this decreases the output of driver but increases its precision.
ported designs on the other hand does not provide any resistance hence does not reduce the output, on the other hand if the port is tuned properly, u can get up to 6-7dB inccrease at some frequencies but if not properly tuned u can have very muddy sound, diameter lenght, geometry and number and location of ports, size and volume of box all effect the final out put so be sure to get a proper audio technition for all this, actaully it depends on ur requirment, u might be happy with just the driver fixed on back seats or u may not be satisfied even with sealed box systems, its very different from person to person.