Mixing Headphones (Help ?)

One important piece of information is missing from your posts as well as your signature, and this is your audio interface.
Most interfaces have a dedicated phones output and is usually sufficient without requiring any seperate headphone amplifier. The Beyerdynamic DT880 are no different than most other studio phones in this regard though it appears to be available in a variety of impedance specs (32,250,600). Again you need to check your interface’s specs.