I've never seen a sub install with the shelf in place under the seats. If you are going to go shallow enough to fit there you might be better off building a box to go behind the rear seats, there is probably enough clearance for a shallow mount, you would just need to make the length and width dimensions quite a bit bigger to get enough airspace. I'm just guessing but I would think there's enough room back there to do this and would be a more viable option.