Behind front seat sub placement

I drive a Toyota Matrix and bought this car for the cargo space, which is very important to me. Mounting in the back is absolutely not an option. I will either be putting my sub on the ground behind the driver seat or passenger seat, however I am not quite sure which way to face it and what effects that would have on the sound. Could anyone enlighten me please?

specs:
GTO804 8" sub
Sealed .35 enclosure
GTO1004 amp feeding sub 300w
listen to lots of styles, but mostly face paced double kick heavy metal.