You have/had an onyx omg bro i just received mine 2 days ago and I was blown away it single handedly made the most improvement in my system out of everything ive ever owned or own. I love it !Seems like Metrum is sending a new USB module to me very soon. Will let you know how that goes.
Interesting information on I2S. The lack of standardization on it makes me not super interested in pursuing it at this time. I think the pinout is most likely accurate. Here's a DDC with an RJ45 out that doesn't match Metrum's. That's about the only one I could find with RJ45 instead of HDMI.
https://www.shenzhenaudio.com/l-k-s-audio-usb-100-usb-audio-interface-pcm384-dsd512.html
All in all I'm probably better off sticking with toslink or switching to USB if the new module fixes things. I get curious because I've seen quite a few people write off toslink as inferior to other connections and I2S and AES seem to get the most hype.
The Metrum rep seemed to be of the opinion that people can argue over toslink and AES, but they're very similar. I assume he meant similar enough that I really don't need to be concerning myself over buying a $400 box just to use AES in the first place.
Still loving my Onyx regardless of which connection type is better. The only way I could maybe be happier is if I could afford a Pavane (not happening)
Ps I currently stare at the pavane lvl 3 everyday.