I compete in db drag and bass race competitions with my car, and have been for about 4 years, just for fun. I have always seen a rise in the db'* facing the rear.
When you face them backwards it lets the sound waves reflect off of the rear of the car and fire forward back towards the passengers. Bass does not travel through metal (which is why trunks rattle, I know mine does). Bass will travel through any cloth, human or wood thats in its way.
If you like to hear bass outside of you car, you can always roll down the windows, but I could car less. I have to spend time tightening bolts in the rear of my car on a monthly basis because they get rattled loose, this does not sound good. I have already had a taillight fall out before I started tightening things.
BTW - I have 2 15s and no trunk space, so you have to give a little to get a little