i would go with a sealed box.
on your amp (Directed??:shrug: ), set your ohms to the lowest response.
set your bass at about mid.
i would set the head unit equalizer with
30% "first slot" bass.
0% "second slot" bass.
3rd & 4th slots are usually for mids and should be set at about 30%
5th & 6th are usually for highs and should be set at 100%
3-6 no effect on subs.
with this set up, IMO you get the best response from the subs and interior speakers!
the effect you want to get is pure deep "tight" bass response and not a "loose" response which 15"s are known for.
i you let down your psngr side rear seat, your gonna get the response you're looking for throughout the car with outsiders hearing a clear response.
Your back seat up will restrict the sound more to the interior
**though outsiders could still hear!!**.
but it also traps that pressure and you will start hearing alot of rattles.