Facing back or up in a hatchback usually sounds really good. I think it's due to the reflection of the sound waves back into the car. But I hear a lot of systems facing forward that sound REALLY good. Also, if you really want other people to hear your music from outside your car, then facing forward is better. But then that just means more bass escapes your car. I agree that with a really good amp, it shouldn't matter too much where they face, but usually facing back or up enhances the bass because it reflects off the surfaces (kinda like how bazooka tubes are supposed to be under 4" from a solid metal surface in order to sound good).

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2004 WRX - WR Blue Pearl
1997 Eclipse GS - Royal Sapphire Pearl