2 12'a can/should be loud/low enough for loud daily music. Save some money and a bit of cargo space...unless your going for spl or trying to win a pissing contest
I always question logic of people who post things like "I wanna do 4 15's, but want it for sound Q, I am not that worried about spl"
For 12's I have seen many happy in the 2-2.25 per sub, 35-38hz tune
Lots of recent post on sundown lately to reference