Tweeters usually fry over clipping of the amplifier, as the tweeter heats up trying to track a square wave. Under powering is most often the root cause of clipping, but not the only cause. The non linear nature of power output and SPL means a small 3dB change = a doubling of power. 3dB is the difference between 50W and 100W, 100W and 200W, or 250W and 500W. You can burn up tweeters in a passive system fast with small power amps and party!
Brad
Brad