I recently purchased the Metrum onyx dac and have a fairly good resolving audio system including the Pathos Twin Towers integrated amp, Green mountain audio speakers and two REL subwoofers. I can attest to what David said in that this DAC sounds very full bodied, natural yet retains excellent transparency. I was also very surprised at the bass control and dynamics which I wasn't expecting. The highs are not rolled off in my opinion. I've been an audiophile for many years and have listened to many dacs and can highly recommend this. My last DAC was a modified Dodson DAC 217 which I've preferred to newer designs until I purchased the Metrum.
I was looking to try a NOS as I've never heard one in the past and was definitely surprised on how good it sounded. There is a 30 day return window so I tried it and kept it. It did sound better to me after being plugged in for one week, after that I didn't notice any change in sound.