Some additional curious things. I hear music with an additional (USB) 24 Bit DA-converter that is only active when the additional amplifier is switched on. It has the name WEILANG 24BIT in the screenshot. When i switch it on it is possible to hear music with it using clementine. But it is still greyed out in phonon! So phonon is working somehow random. (In Debian 8 it is working much more stable.)