If you wanted to see for yourself why NOS DACs are poor choices, look no further than this post of erin's. View the actual post to see the attached pictures.
the following attachments show photos of my CRO screen when generating various tones in foobar and running into the Valab through the USB.
The first photo shows that with the volume set to 100% in foobar that the negative half cycle of 40Hz is clipping. The clipping was eliminated when turning the volume down about 3dB.
The more concerning issue is what I saw when I incresed the frequency of the test tone whilst mainatining the voume at -3dB.
At 5Khz the sine wave(hmmm) is not a sine wave, it appears to be full of harmonics or I was thinking that it looks like there could be a delay in the i2s data lines which is resulting in the DAC's outputting different or delayed data??? - just a thought, no proof of this, maybe someone else could shed some light?
At 10Khz the waveshape is undescribeable.
22Khz looks as expected.
Well, this doesnt take anything away from the way people percieve the sound quality or take away from the enjoyment of the musical reproduction, but the weird stuff happening above 5Khz speaks for itself.