Hi Fredrik,
NAD d3020 + Raspberry Pi (B)
So you have a Pi1 (first model)?
If so, the image for that device is not capable of playing native DSD.
If not, the new image for RP2/3 can play native DSD to USB DACs.
For this to work the MPD configuration has to be tweaked by hand.
This will change soon with the next image.
Native DSD does not work with all DACs see
https://github.com/lintweaker/xmos-native-dsd
Frank