Do you guys have your subs in the trunk facing the back of your car? That's how they installed mine the guy said it sounds better that way. I don't understand the science of that.
when I was in tech school in the Coast Guard, they explained that sound has a "path of propogation." For amplified speakers, that path is roughly 13 feet, so that's where the idea comes from that facing backwards is the best way because the distance the sound travels going away from you towards the back of the car, then back to your drivers seat is anywhere from 5-10 feet or so. Also part of the reason that the sound is louder in front of the car sometimes than it is behind it (even if the speakers are facing right at you if you're standing behind the car with the trunk closed.
This all depends on the vehicle, speaker box, etc... In my experience, if it's a hatchback, face them up directly at the glass. In a truck it almost doesn't matter, etc...