Unfortunately, this DAP wasn't really designed to run moderately power hungry HPs or IEMs and above. I believe that's why they made a metal version with upgraded opamps and chipset to better handle some other gears. IMO, this is one of the very few weaknesses of the DAP, its driving power. But nevertheless, as long as you plug in an easy to drive HP or IEM then there wouldn't be a problem at all. For me, I use high gain only when using my K7XX HP (which still wasn't driven properly) and TY Hi Z earbuds. For IEMs I have no problems. I use middle gain only.
This DAP in underrated. Many other and newer DAPs are getting attention because of their feature sets and innovation. But for the Opus#1, you won't be disappointed with the SQ at its price point.