It's the box, i couldn't get my 15's loud either, so i decided i'd just go nuts with some test tracks, and see if they'd blow up......
Put in that technotronics ft. bass mechanik song from realm of excursion, turned my sub-level to +5 (my gains are set at 0) and cranked it, and HOLY CRAP, me and my roomate were just laughing at how bad it hurt not only our ears, but my entire body ached for an hour or so after that,, but then i turned it down when i started smelling coil (subs are inverted, they weren't roasting more than 3-5 seconds)
The problem's definitely my box...... My port's too small, and the box could be abit larger as well.... I may build a 6cf box for just one of them with a BEEFY port, and see if that isn't more pleasing to my personal requirements.
Not only size of the box, but size, shape, and placement of the port are very important, i'm learning. With the correct box, it's possible to get full xmax movement of the sub with less than maximum power, so long as you have the space.