I spent 5 minutes looking around and Focusrite didn't mention a headphone amp specs, which means its probably nothing special and wants 32ohm standard headphones.
You'll probably need a outboard amp for 300 ohm headphones to get decent volume, but I assume you already have read the same searches and comments, going back to 2017 ….with HD600, anc Clarett people tend to notice low output.
1) buy a headphone amp
2) buy different headphones
3) get a different interface