The subs sound great. I've had them facing the seat and facing down. Each sounded different, but both great in their own ways. Towards the seat was louder, but facing down was cleaner. I've never had them facing the tailgate, since i put so much stuff back there, so it really wasn't feasible... so i can't comment on that one.

IMO, a properly designed box makes a huge difference, enough to overcome any downfall of positioning.