spdif vs upnp vs nas

Hi, i recently bought a naim unitiqute and when i played songs through its usb port (which can only host limited capacity usb sticks) it sounded phenomenal.

then i configured Asset upnp and played the same music but felt SQ was not as good as USB.

i have not tried out a NAS yet.

So would like forum-ers opinion on which - spdif or upnp or nas - would have the best SQ (when used with unitiqute).

I have the unitiqute hardwired to my internet router btw.

And my primary goal would be to play uncompressed FLAC which are on a external hard drive.

Appreciate the insights.