Didnt you say you can't fit a 15"? If you can fit a 15", the single 15" will get louder then the 2 12"s no doubt. More people should be running single 15" trunk setups for what theyre trying to achieve out of their daily cars. Unfortunately, local marketing strategies and word of mouth says 12"s are the chit. If your friend says 1 15"
CONE AREA.....
1-15= 15 INCES OF CONE AREA....
2-12's= 24 INCHES OF CONE AREA....
2 12's will push considerably more amounts of air than just a single 15...also handle more power together than one 15....