Since i am a user of Volumio for quite a while i like to share the following with you.
I used my Volumio with a Hifiberry Dac hat and the sound was great.
It took its analog output to my D3020 amplifier.
Since there is a DAC onboard of the NAD i wanted to try that one out as well, so i bought a Hifiberry Digi with transformer.
Hooked it up at my raspberry with Volumio and connected to my NAD by coax.
Turned my music on and… i got very excited!
The sound was much better, the bass was tighter, the voices clearer, the complete soundstage was excellent.
I always were enthusiastic about my little NAD, but this made it even better.
Do you find the Hifiberry is much of an advantage?
I have been driving my NAD D3020 direct from the pi and it sounds pretty good to me.
If you mean driving through USB you really should try the Hifiberry.
It’s the best upgrade for your hifi for a few bucks.
And hey, if it doesn’t work out, you can always return to USB, but i assure you: that is not going to happen. 
In order of sound quality for what i have tested:
- HifiBerry Digi over coax through NAD
- Hifiberry Dac over analog input NAD
- Raspberry Pi over USB input NAD
My initial reaction to mveldman’s post was that it shouldn’t matter if coaxial or USB is used-- it’s the same digital signal. Yet, on more research, it can matter as it has to do with the clocks used for timing where SPDIF leaves the timing to the DAC and USB will rely on whatever quality clock is on the board computer. As DAC is dedicated to sound, using SPDIF can reduce jitter and other anomalies that may occur with USB. Hmm… time to start considering the HifiBerry Digi+…
head-fi.org/t/623204/digital … ifferences
Don’t take my word on it, just do some listening 