I have had a SGCD and a pair of M700 amps for about two months now. I love the sound of digital files via the SGCD. All my 1100 or so CDs have been loaded onto a computer running Logitech Media Server which I have been using since the days of Slim Devices. My inputs to the SGCD are a Squeezebox Touch and a Wyred4Sound modded Sonos Connect.
I purchased a NuPrime CDT-8 to play CDs and I wanted to try out the I2S connection. The I2S connection only works when I have sample rate conversion (on the CDT) set to DSD 64 or DSD 128. It does not work with sample rate conversion turned off or set to any PCM value.
I also tried using the RCA coax connection. This works fine with sample rate conversion set off or set to any PCM value up to the SGCD limit. I did not test the optical connection. I decided to go with the I2S connection. It sounds great and I just have to decide between DSD 64 and 128.