I recently took delivery of an Audio-GD NFB-2 for use in my system consisting of 14 year-old Creek amplification and CD player/Epos loudspeakers/DNM Reson interconnects and speaker cables. I've always known this setup to sound very smooth and flowing, without being too hard on the ears. I feed this with output from Fidelia on OSX through either USB (Wireworld cable), SPDIF from the CD player, and Optical from the Apple TV2 which takes a WiFi feed from the MAC.
My initial ratings of the sound were:
1) SPDIF from CD player (much better sound than analog from the CD player)
2) WiFi/Optical from the Apple TV2 (also better than the CD player)
3) *** USB (not good at all)
*** I recently tried a Kingrex UC192 in the system to take the USB feed from the computer, and then pass the signal as SPDIF to the DAC. This sound has been wonderful, and now goes to the top of my list by far.
Incidently, I had previously tried an HRT II+ in this system, and found things to be very flat and narrow sounding. My wife said, "I can't listen to that, turn it off", and that was all I needed to hear to know that the HRT II+ was not for us. I suppose the key to this is looking at the differences in circuitry and power supply between the NFB-2 and the HRT II+.