I2S Recommendation

What input best depends on how well each interface optimized. The Harmony usb interface nothing special so its very possible I2S will provide superior sound quality. In my case I was using Sonore OpticalRendu with Audioquest Diamond usb with Harmony at first. Later went to OpticalRendu feeding Denafrips Gaia usb, Gaia to Harmony via  .5m*Tubulus Ximius, Harmony sync'd to Gaia's superior I2S clock. 

To really take advantage of I2S potential, one should have highest quality OXCO clock in streamer or DDC vs dac internal clock, so superior timing via syncing to the external clock. Also keep I2S cable short .5M or less, longer cables throw off timing defeating much of the advantage of the external clock. 

While the Harmony was nice enough via usb, going this particular I2S route really upped the game, greater resolution, transparency along with more analog like presentation.

@audiostick What I've learned after more than 10 years streaming is silver cabling superior to copper, this with ethernet, usb and I2S cables, the higher the silver content the better. Using all Audioquest Vodka for ethernet, AQ Diamond for usb, and of course the Ximius for I2S. Haven't used the other Tubulus cables. Silver is a better conductor than copper, I hear greater resolution, transparency with high content silver. 

I don't know there is a sound quality penalty paid for using HDMI cable vs I2S, but you're paying for unused runs of wire, in the case of pure silver this can really run up the bill.