Given the size of the diaphram, and the amount of heatsink It seems reasonable to think that you
need more than 5 watts rms to damage the headphones, probably a lot more than 5 watts.
However if you have a much lower power amplifier, and its clipping very hard into the headphones
then the real power being delivered could be way more than 5 watts, and do even more damage.
So you need an amplifier that won't clip, but also does not have the voltage swing necessary
to do damage. On these headphones it seems to be a fine line.
Agreed, it is much easier to damage an (ear)speaker with too little power than with too much power.