I cant imagine too much air flowing out of the spare tire well on an E53 (or anything that isn't a Dodge product). Whether load floor or not, the floor is made specifically to add support to the rear shelf for hauling and insulation from road noise way down there in the Bimmer nethers.

My 1st hand (ear) experience (ear-perience?) is with my own JBL Stealth box and a JL 500w amp. I've run it now for 3 or 4 years (maybe more?). I do use Focal's in the doors, so I have pretty decent quality there. They run off of a JL VX 800 which does a fine job as well.

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