Question about Beyerdynamic DT 990Pro

with reservations, yes. according to the answer already given. Not all 0815 devices have sufficient power for headphones with high impedance! my son uses these headphones with a Samsung A 72 (or something like that) and the volume and sound quality are always sufficient for real pleasure. With a high-end player/amplifier, the volume and sound are of course much better. Well, for that the DT 1770 and 1990 or even better should be available :P

Yes, the 250 ohms might be a bit much for the mac mini directly. So if you want the headphones to be really loud, you might need an amplifier.