No it's measured in much the same way as as it is on conventional speakers and subwoofers. some subwoofers have absolutely mentally rated power handling when in reality they only need a fraction of it before they reach their mechanical limits. in those cases the enclosures the drivers sit in play a big part in it. Sealed subwoofer, for example, will take a lit more power at low frequencies than the same driver would in a ported box (below the tuning point of the box that is) because the pressure in that sealed box acts as a natural spring. the ported box doesnt have this and so the driver is a lot more 'free' and power handling decreases as a result.
headphones will be much them same, take a pair of free-air (open) headphones and run a 15hz tone through them. you'll heard the drivers cracking up and hitting the end stops (sounds like crack or a loud pop) long before they reach their quoted power handling. this can be proven by running a 50hz tone through the headphones at the same volume level as you wont hit the mechanical limits and chances are you'll be able to turn the wick up quite a bit before you hear any obvious problems. The hd650's are good for this - they arent the most efficient headphones below 30hz and the drivers a re too small to handle any real amount of power. if you arent careful, they run out of puff well before they really should do. i run a filter on everything below 20hz because of this, just as a precaution

These points are all generalisations, of course, but they explain enough hopefully.
I thought that maximum input power was the power at which the headphones maximum volume was reached
No, definitely not, if anything its as ambiguous as RMS ratings are in general. it's nothing something you should use to base any assumptions on, promise lol.
100 ohms is a funny 'middly' impedance. It's a level where many cheap amps will be either current or voltage limited or both.
Well, this is it really. some amps just arent suitable for either application, at least not seriously.
Futher edit: I have never found volume to be a problem for the passive NS1000s. It's just that sound quality takes a real dive with no amp or an unsuitable amp.
I havent either
reaalllly, at least not a proper headphone amp. I found my old D2 absolutely comical with my 650's, but the old thing im using now is ok. What it isnt up to scratch on is everything else - background noise, seperation,
control, which is something entirely different to power handling..it does the job but there's room from some serious improvement.