I didn't see, is somebody try with very "hard to drive" headphones like HiFiMan HE-6? Does U-05 have enough power for HE-6?

well, from all HiFi component, headphones are more subjectiv. When I tested Senn HD650, my reson to give up of them is that they are (please, don't laugh) small for me (better to say, my head). They build from metal which is not flexibile.    When I bought 325, in first time I have problem...

I will try to keep then, and add another Grado (or HiFiMan)

my 325is No: 9391 (with G cush) running on: Musical Fidelity x-cans (with 6n23p-ev tube, Panasonic FM caps, UF diodes...) Cambridge Audio DacMagic m2tech hiFace   ps frst Grado was 60 (short), second 80... I try Senn 535, 555, 650, AKG 141, but, Grado is best for my head and ears