So, now I tested them, it works, but as expected, it’s not very loud. It might work at home, but not very impressive. The Beyerdynamic T90 with 250Ω works quite well, although not as loud as a 32Ω headphone like the Philips Fidelio X2 for example.
So I still think, there is no real limit for the impedance, it depends on what you like to do with the headphones. For a mind blowing, ear shattering volume, you will always need an amplifier.
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