Been running one of these since getting it via Massdrop a couple of weeks back. For the price it's an excellent unit and has been getting better as I leave it on. I do find a big difference between the optical and USB but that was evident with other DAC's I've tried with my system. The optical has greater definition and space around the instruments but can be grating up top, while the USB is fuller on the bottom / mids but loses some clarity and articulation, especially in the top end. What I wasn't prepared for was the difference in USB cables I tried. The stock cable was pretty good albeit a bit "thin" sounding in comparison to the Schiit and Pangea cables I tried which imparted a fuller and warmer sound. Now I wonder what stepping up to a Wireworld Starlight 7 USB cable would do? Bottom line is I would highly recommend this DAC to anyone looking for a quality unit at a very good price. One day I'd like to own a Ayre, Aqua or a Audio-gd Master level DAC but for now this little M8 is good enough, and even better than that.