I have the M21 and very happy with it. Mike at Audio Archon sells both but recommended the M1 which i originally purchased. I decided to go ahead and upgrade to the M21 to benefit from the analog volume control. I was glad I did. The M21 is worth the extra coin. I use a PS Audio Airlens as a ROON endpoint to process the streaming. The M21 has better power supplies than the MPD-6 at least from what i can observe looking at open chassis images.
