Mine is giving 19.2V. On medium gain i can push 100% volume pot fot both Hifiman HE400 or Senns 650 with no issues.For me it was while driving a set of Sennheisers (HD565) and also happened with DT990 250ohm, one other time nothing was playing went for a coffee came back and the LED was flashing with no sound output, almost always have it on medium gain volume normally no higher than 1/2 oclock position, should be more than capable of that.
Had a check on the 15V AC with a multimeter, seems to be working showing around 18.5v