I see, a little on the low for my DT 880 Pro 250 ohm, might be enough for my Amiron.
I don't understand how a smaller and cheaper Fiio K7 can output 560mW at 300 ohm.
Low? Certainly not in power, because this headphone is spec'd at 100 mW power handling capability. Most headphones are spec'd lower.
At a nominal SPL of 96 dB at 1 mW the max "safe" (for the headphones, not your hearing!) SPL is 116 dB. Turn up the volume beyond that point (100 mW or 116 dB SPL) and you risk permanently damaging the drivers anyway.
As usual power is not the issue. The 32 ohm version is much harder to drive, for example, as it needs 3x the current for the same SPL.
People keep confusing power with lack of
gain, which typically is the actual issue.
In my listening level spreadsheet I calculated that in the worst case with completely uncompressed high dynamic range classical music you only need ~24 mW for "safe" (depending on your listening time) average listening levels. And this is not continuous power but will only be required in very rare peaks in the music.
If you listen to modern pop/rock then you only need ~1 mW for the same average listening level.