I tried driving them from the speaker outputs of some 20 year old Jeff Rowland model 3 power amps. They worked fine, using an Audio Synthesis ProPassion passive preamp, although three notches on the volume controls was enough iirc (virtually nothing). The sound was smooth with no noticeable noise, but it lacked drama, flair and drive. I was underwhelmed and went back to my headphone amps. I didn't use them on the power amps for more than an hour or so as the results weren't interesting enough, so I can't comment on whether long term damage might be done. I just had to try it as the idea had been bugging me and the amps were gathering dust.
Damage should only occur if you turn the volume up too high.
The power used by the headphones will be the same at the same listening levels on any amplifier.
The only exception is if the power amplifier has a significant DC offset which may build up more heat.
In this case, when not playing any music, the earphones will go "click" when plugged or unplugged.