Poor THD+N performance on Khadas Tone

I have been playing around with a new Khadas tone board (V13 2018) using USB interface.
On the Windows computer I used ASIO4ALL and the XMOS driver provided by Khadas.
In the R.Pi I tried all the settings I could find including many different resampling settings

In my computer it performs very good, but on the Raspberri Pi 3 B+ there is increased noise between 5kHz and 20kHz as can be seen on the picture below.

I use a Rohde & Schwarz UPV Audio analyzer for measurements. It is a high performance equipment.

Suspicion of noise from the R.Pi power made me isolate the USB bus using a ADUM4160 USB isolator powering the Khadas from a Li-Po battery regulated to 5V. This made no difference.

The noise must originate from the way Volumio configures the XMOS/DAC.

Maximum volume

The noise level is at -105dB and is probably not very audible, but this is something that should be addressed.