i run a 10" sub in a 1.2 cubic foot ported box that plays low and loud. it leaves plenty of room in the trunk for 2 big amps and "stuff."
i've got a couple of super efficient nakamichi "free air" subs laying around that kick ass in a sealed box and play really loud with a low power amp. i've got a pair of nakamichi 8" subs, same version, that i'll keep forever but the 10s could be up for grabs.
i also have 300w and 500w hart professional, high current amps on the shelf, too. they're smaller versions of what are in the above pic. they're old school competition amps and are rated way lower than their performance.
i just say no to junk car stereo.