Clipping is the enemy, here. When an underpowered amplifier "clips" or runs out of steam and the power supply can no longer provide enough current to the output tubes or transistors, or when those devices reach their maximum output voltage, the signal gets clipped. The waveform of the output then contains a huge amount of DC voltage, sometimes at very high voltage levels relative to the levels seen when the signal is behaving itself as AC. This DC voltage (and the other trash thrown out by the clipping devices) is what does the damage. The voice coil of a speaker is actually a motor. When it sees DC current, the motion of the voice coil stops, or "stalls". When this happens, instead of acting as an inductor, the voice coil basically becomes a piece of wire, and the amplifier sees that piece of wire as a dead short. This causes a spiral of death. The short demands more current from the amp, which it can't deliver, and it clips even harder. Meanwhile the voicecoil is heating up from the increased current, and the lack of any cooling fanning motion, and poof - puff of smoke, and no more voice coil. Even a microscopic burn can sometimes kill a voice coil. This is why, when used and applied properly, it is always better to have more amplifier than needed. Clipping is much more destructive than a small amount of overpowering. One last note - loud is not always better, but the only way to get really loud is to have a larger driver, larger box or more boxes. A bookshelf teensy driver and billion watt amp is never going to push the air mass that a big box and an appropriate amp will move.