Hello James,
Thanks for your first impression.
I am quite curious what your tests will show.
With the M51 I asked a NAD technical specialist at the Munich High-End show 2015 about thr HDMI connection, and the answer was, that when using the OPPOs 103 "audio-out-HDMI" connection with the M51 input, a I2S connection is established with a minimum of jitter in contrast to the SPDIF or AES dig. connection....
So at least as I did unterstand it (I am no technical expert....).
Greetings from Germany,
Alex
P.s.. the HDMI connection is "nice" for transmitting SACD, BLU-RAY..... if it does sound good.