I just bought a new M51 for 1 grand on clearance. I'm running newest FW 1.50. I'm about to try revert to FW 1.39. I'm scared schitless that I might brick my unit. For anyone that doesn't know, FW 1.39 has 1dB more gain than the newer firmwares, which is why many people found it to sound more dynamic and lively than newer firmwares. Nad reduced the gain by 1dB due to clipping when run at 0dB volume, but that was easily fixed by running at -1dB volume. So technically the newest firmwares are a better implementation of the hardware as 0dB is bit perfect for the dac and should not have clipping, but to achieve this the dynamics will have taken a hit due to the reduced gain.
Also on a technical level the SPDIF inputs of coaxial and optical should be the most pure representation of your source input, as it takes PCM from the input and gives it straight to the M51 Dac (Zetex DDFA) to convert to 844mhz PWM. While the other inputs go through another chip before going to the Dac, the Usb input goes through an xmos chip, the Hdmi input goes through an Analog devices chip, the Aes/ebu input goes through an Akm chip. So all these chips introduce their own sound sig onto the input signal, not that its a bad thing as they are all high quality chips, but the SPDIF input will just have a sound that is truer to the source input, I also always favor coaxial over optical in any system.
Thanks for the technical info on each input. I just bought a new music server and it has usb output only.
Is it gonna be a lot better in sound quality if I bought another usb/spdif converter to use with NAD M51?