I've been using the new build of Poweramp on my HTC 10 and, when listening through my Oriveti Primacy's which are quite sensitive, notice a fair amount of hiss to all songs. It probably isn't the amp in the phone as the hiss is only present when the song is playing and increases in volume with the song, perhaps noise in the file or the way the file is processed? I'm not sure. I did noticed however, that toggling the Hi-res output removes all output noise and the phone produces an essentially black noise floor. So evidently the Hi-res option is doing something significant, however it's still pretty unstable, often failing to output after changing a few songs as a few others have mentioned.
I love the look and features of Poweramp and have purchased the full version on numerous devices. Do you have any plans on further developing this feature? It really improves the sound quality at the cost of reliability.