Ok, I found some minor issue: apparently audio routing to USB does not work. I tried to feed an external DAC with the XZ3 via USB. When the DAC is plugged into USB, audio is still routed to the XZ3 speakers.

When I do the same with the XA2, audio data is routed to USB and thus to the DAC. The XA2 speakers remain silent when playing music and the DAC is connected to USB

EDIT: Audio-out to USB is fine as far a the analog signal is concerned. Thus using the passive USB-TRS adapter works fine The issue only concerns the digital audio data

And: USB OTG also works fine - USB sticks are well recognized