First off, why is this in the LCD-2 thread?
Second, the 727 has 350V rails so it can't really produce enough voltage to kill the drivers.
Now there are two main ways to destroy electrostatic drivers, over voltage on the stators which shorts out the D/S gap and causes an arc and then there is simply destroying the high resistance coating on the diaphragm. Now the first one is simple, keep the max voltage swing below bias*2 and you should be good. This is voltage peak to peak so roughly 1200V for Stax pro bias. This is insanely loud but the BHSE, Carbon and KGSSHV will easily reach those peaks but you also need a very high input voltage.
The second part is a bit more complicated. Now for all intents and purposes, there should never be any current flow on the diaphragm. It should hold a charge but said charge should be a constant and never change. Now we live in the real world and it simply doesn't work like that but the charge has to be carefully controlled as little current flow as is possible.
When we design amps we account for these things so none of our amps should ever damage the headphones if they are built correctly. We can't control what others do but if follow our lead then they should be safe. This brings us to the people who really don't know what they are doing aka Woo Audio, Cavalli and RSA. Those amps aren't safe and in some cases are guaranteed to destroy your headphones.