If you were to put - 5W speaker to 10-15W amplifier which - bass or treble will most likely damage it first? From what I've heard and learned - boosted bass makes speaker move back and forth and if that moving energy is too big for the speaker to handle it may rip. On the other side, boosted high frequencies generate more heat in the tweeter and it may overheat*.
One more interesting thing I found, is that it is generally better to put a 5W speaker to 10W amp instead of 5W speaker with 5W amplifier which is starting to clip because square sound waveforms (distortions) put much stress into a speaker and it starts generating heat if the clipping is too much. It may even catch on fire! While 10 clean watts give 'natural frequencies which don't put as much stress (peaks) and is easier on the speaker even though it's louder.
I think this is an interesting topic to know about.