Top firing vs down firing sub setup

I'm working on a rear seat delete and reworking my car audio in the process.

So I've begun work on the sub box/amp rack which will be forward of the strut towers and occupy the rear seat area. The goal is to keep everything the same height as the original floor, or close to it.

Top and bottom pieces of the sub box/amp rack in progress:
http://imgur.com/cBOrp5q.jpg
http://imgur.com/HJaZcX8.jpg

Now I've been trying to decide on how to mount the sub(s). I'm using Alpine PDX stackable amps which are indicated in blue in the 3d models. Sub(s) are indicated by green.

One idea is to mount a single sub in the top and have it fire upwards.
http://imgur.com/JDlkgEa.jpg

The second idea is to mount 1 or 2 subs in the bottom and have them fire down towards the sheet metal of the car. The area in red would be a "vent" to allow for air pressure to escape from the area under the box. I would put a speaker grill in the red area to cover the hole. My understanding of down firing subs is that as long as there is a vented area comparable to the sub cone area and enough room under the sub to breath it should be alright.
http://imgur.com/Ckd3008.jpg
http://imgur.com/sEFhAZ9.jpg
http://imgur.com/nrxIool.jpg

So, does anyone know if down firing like this is a good idea? I like the idea of the sub(s) being protected and hidden but I'm not sure if it will just cause a bunch of rattles, sound like crap, etc.

Down firing is just fine, make sure that the cone are not being trusted into the floow while playing music.