Is DT880 600 ohm with 100 ohm an industry thing?

If so, Then get an amp with around 7.75+ volts into 600 ohms with a 100 ohm OI

That’s approx 100mw into 600 ohms raw RMS without the complex impedance curve of the headphone and the interaction with the OI of the amp

You can drive high impedance headphones with low Ouput Impedance amps but the higher output impedance options are like a nice analog EQ very tasteful sometimes