I would have to say it really all depends on the type of 15's you are thinking of going with. My buddy has 4 sa-8 subs on Kicker KX2500.1 and it is LOUD!
They are under the rear seats of a Toyota Tundra. I'm sure they would be much louder in something like your X-Terra.
Again though, really all depends on the 15's you're thinking of going with