you will definately have cleaner bass with the 12s in a sealed box. It will be tighter and hit harder. I got a friend who has two twelves in a 4 and he does not have near the power of BD1000 adn his shit hits mad hard and can be heard from pretty far away.
Dont get two fifteens unless you are a filthy basshead like uhhh me

. I only have one and it is sounds better inside and it almost as loud outside the car as my friends JLW7 12" (powered actually by a BD1000). But i neeeeed another one, one is not enough for me. 8)
BTW before i had my 15 i had a rockford HE2 DVC 12" in a ported and it sounded great. Rockford makes good stuff and you cant go wrong with the bd amps they put out what they are suppopsed to.
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Pioneer Premier, Elemental Designs, SoundSplinter, Resonant Engineering Audio, Stinger, Optima, Rockford Fosgate