I’ve had bit more time to use the amp, and it seems the headphone output is sensitive to what headphones are used. I bought a pair of KZ ZS10 Pro In Ear headphones, and I got weird distortion and not great sound. They have very good frequency response according the specs. Same happened with a very decent pair of Bose noise cancelling in ears. When I use my high end over ear headphones, all is fine. I reckon it’s a drive/impedance issue, but haven’t yet checked the impedance of the three pairs I’ve tried with the amp.