Headphone impedance is also only a part of the story, amps have none 0 output impedance, though for most of the cheap solid state ones it’s so low it can be ignored.
Higher impedance can be a good thing for amps with higher output impedance (like valve amps or low feedback solid state designs).
In effect the amps output impedance acts as a resistor in series for Voltage load and a resistor in parallel for Current loads. Depending on the drivers in the headphones, you get a bit of both.
It’s the primary reason that Valve amps don’t generally like planars.
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