Well not only is less power needed to get high output but less power is needed to drive the sub past its excursion capabilities and destroy it mechanically. Sub power rated power handling is only half of the equation. It only covers the amount of power that the motor of the sub can thermally dissappate for an extended period of time (forever, theorectically). The othe half of the equation is keeping the sub from reaching its excursion limits and being mechanically damaged. This is entirely related to the enclosure, both volume and tuning. The larger the enclosure, the less control the enclosure exerts on the sub. This lack of enclosure damping also lends to the efficiency of the enclosure. It can get louder with less power, but it it can't handle as much power either. The limiting factor is the ratio of the volume of the air moved by the sub to the overall volume of the enclosure. In a really large enclosure this ratio is really low. Effectively the enclosure will reinforce the sound from the driver via the port but won't aid in the control of the cone. Final result is the extra output but the sub acts like its in free-air except right at the tuning freq where the inertia of the air moving through the port increases the effective ratio of moving volume to the enclosure volume.
