I got my m9xx from the original Massdrop sale. I was having a lot of issues just recently where it would randomly just stop working (power would stay on, display would work, but the unit would not output any sound unless both the power USB line and data USB line were disconnected and reconnected). After struggling with this for about two weeks thinking it was drivers and possibly even a stupid windows update issue I decided to try out a different power supply. Since switching to another USB power supply all my problems have been fixed.
So if anyone has issues that are similar to the above just change out the power supply.
Love the DAC though. It has been such a strong unit. I recently bought an Accuphase E-650 integrated and I hear that the headphone section in it is truly amazing. So I will be comparing it to my m9xx and see which one will be my main headphone powerhouse. Luckily, the DAC section in the m9xx is great and has very good support from Grace, so I can still use the m9xx DAC going to the Accuphase if I want.