Echoing what others posted, in my way of thinking of it-
Headphone amplifier must produce voltage swing necessary to produce desired peak headroom above desired sound pressure level. In order to be a constant voltage source with respect to load impedance the source (amplifier output) impedance would be much lower impedance, perhaps 1/10 of the load impedance. That source would need to be OK and stable working into the capacitance and inductance present in a load impedance (as opposed to a pure resistance)
So... for 25 ohm impedance headphone, I'd want an amplifier with less than 2 ohm output impedance, with the ability to produce desired voltage swing into 2 ohms, given the efficiency of the headphone to produce desired sound pressure level.
Back in the '70s and '80s we used "hi-fi" power amplifiers in the lower power ranges 25 to 50 watts to drive studio headphones, which at that time were usually very low impedance. As higher impedance headphones such at Beyer DT-100 (etc.) series came along, the same amplifier could be used to drive several pairs in parallel. Each headphone could even have a stereo pot of say 50 ohms to 100 ohms to control level. Not technically perfect because the source drive impedance is increased, but stable and workable in a studio environment.
And talent is unlikely to damage the amplifier, regardless of what they plugged in.
I think the big thing now is getting the voltage swing into 25 ohms with a USB powered headphone amplifier that is subject to power limitations of a host USB connector. Some may say simply buy higher impedance headphones. However, that limits your choices.
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