if you really want to be different and put ur sub on your parcel shelf then go on
www.caraudiosecurity.com they do on there reinforced parcel shalfs that can stand a lot of weight. this is because of people like you who have crazy ideas. the only problem would be the bracets but i mean make them stronger with some wood and you would be fine.
if you do manage to do that can you post some pics to me as im very interested.
cheers