After several years of break, recently I have re-discovered an audiophile in myself. Needles to say, I am completing my set of gear from scratch and have no alternatives for reference. I wanted the dac/amp combo to drive Denon AH-D5000s, as I felt my PC's soundcard was not doing them any justice.
Luckily I was able to find a local store that stocks quite a lot of high-end equipment and managed to listen to several amps within my price range. The NFB-5.2 sounded as good as some amps twice the price, so I ended up getting it.
What I took home was the display unit that I listened to - that gave me a considerable discount, but also the suspicion that the set is incomplete: apart from the power and USB cables there was nothing else in the box, not even the optical cable that Audio-gd typically includes. Looks like it's the vendor's fault though. But in reality nothing else was needed: I plugged it in and that was it: Windows 7 has recognised it instantly and did its thing without needing any drivers.
The difference it made is worth every penny: the sound is spacious and detailed, the amp doesn't produce any audible noise or distortions even at deafening sound levels, the dac works very well with all the sampling rates I could find, including 24/192. The only drawback is that it makes all the recording flaws more obvious, especially distortions introduced by signal clipping.
I have re-wired also my active speakers to feed off the line-outs. Even that made a massive difference that was instantly obvious, even though the speakers are quite ordinary.
Inputs (USB/optical/coax) are selectable from the front panel. I have tried them all, but couldn't tell any difference. This may be handy if one has multiple sources, otherwise whatever works.
Overall I'm very happy with this combo: it does what I need it to do and does it very well.