From my experience, the DAC in my NAD was preferable to that in the squeeze box. The NAD had a smoother more enjoyable sound, though the Duet had a bit more detail at the expensive of harshness.
On a side note, I love my Duet - it is easily the best piece of gear (of any hobby) I have in recent memory.
On a side note, I love my Duet - it is easily the best piece of gear (of any hobby) I have in recent memory.