Having been able to spend a little time testing the K50's digital SPDIF output to my Bartok, I'm inclined to agree - a short amount of listening seems to indicate the K50's digital output sounds better than ethernet (which is a bit nuts as the Bartok streamer should be the dog's dangly bits, given the reviews, its price and relative age).
I need to wait for an RCA adapter to arrive to test the same output with the 818 and have ordered an AQ Diamond AES cable at the same time. Will let you know what I find... but I suspect the K50 will win (whether a Bartok with the external Rossini clock would better the K50 might be another test!)...