An RMS rating and a Peak rating (max output) are seperate measurements.
I'm well aware of the difference between RMS and max. PAs are rated often for e.g 700W/ch max RMS output.
SRX725 - RMS 1200W - peak 4800W.
Ignoring for a moment that the majority of amps don't have power supplies capable of putting out 4800W and that it requires a 40 amp or greater circuit to even get that out of the wall which is extremely uncommon, an amp capable of e.g 400W/ch or even a slightly beefy one capable of 700W/ch can't get anywhere near killing these. Give them a 1000W amp and you still can't kill them.
The reason its a very bad idea to underspec a power amp is because if a signal with a large dynamic range is being played and the amp is pumping out full throttle, the signal is going to brickwall and clip
Clipping doesn't happen in well designed amps because the opamps won't be overdriven. Tube amps can't even clip, they only will compress as the signal becomes higher and higher over what they can handle.
Running the opamps in a clipping situation is bad for them because they can overheat and die, but again a well designed amp is designed to not die.
If its clipping for a long period of time its not only recreating harmonic distortion in your speakers (the bad kind) its interally cooking the insides.
Square waves are full of HF content. The idea that square waves kill speakers generally stems from the fact that tweeters are much more efficient and more fragile than woofers. As such, square waves send far more HF content into the tweeter than you normally would, which may fry it. This isn't an issue on a well designed and well specced PA. No kind of distortion is bad for a speaker to play. All signals are the same to a transducer.
The voice coil cooks under normal use just as much as a square wave. Square waves simply contain more energy per unit time than a sine wave, so they cook faster. This is still not an issue with PAs which are well cooled.