I found a box already made for them and by "carry sound", I mean you could hear me down the street or inside a building. A guy at my school has 2 kicker comps in a single cab sonoma in a sealed box. Right next to it, it doesnt sound amazing, but he drove by the school and it was shaking the room!
yeah do 2 15's ported. It will save you cash on spending more $$ on 2 more subs and maybe another amp or w/e. Do two really good quality subs on a decent/good amp and it will be as loud, or louder.
I'm all for 2 15s ported. more pressure inside means more pressure outside. but you shouldnt be looking for outside sound. every car is differnt. my car with the SI MAg 15 you can barely hear outside bu my friend has a pontiac grand am with 2 12 type S's sealed and it sounds way louder outside. but of course my Mag is louder inside the car. his car is just poorly dampened. one of many differences between 18k car and 50k car. lots of newer models are getting better with that though.