I used an IQAudIO DigiAMP+ with fully balanced headphones (you MUST HAVE FULLY BALANCED as a common negative [-] type headphone is not supported). I use Opposite PM-2 with the balanced 4 way XLR cable.
Its built on Raspberry Pi 3B and DietPi build, and I have a switch to allow for speakers as an option. Had a post with some pics somewhere but can’t find it now…not portable tho