It is not underpowering the speaker that "blows" it, it is either sending too much power to the driver, or sending a clipped signal (for an extended period of time) to a driver that will cause failure. Ususally, though, if one has an amp with too little rated power, they end up trying to push more out of the amp than can be done cleanly, thus sending a square wave to the sub. Correct ME if I'M wrong.
