In a sub install manual I read that you shouldnt have a thick surface between the sub and the rest of the car. It said use:

-An open middle back seat armrest with hole

-A hole in the parcel tray with the sub facing upwards (could be an old speaker hole).

However I've never seen someone do that before, how important do you think this is?

People just have their sub facing outwards.

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THINK OF THE BIGGER PICTURE