most people get the loudest subs by facing them directly towards the back of the car. it creates a sort of a like echoing effect and bounces off the rear of the car and reflects back. you hear the sound bouncing off the back more than you do it coming from the subs. the trunk acts like a second box kinda thing. its hard to explain. this is just what i have been told in the past i dont know if its correct or not. best bet is to just try it and see what you like.
In my Firechicken, pointing a sub in the cargo area (very back) up is the best for SPL and SQ both.