Actually it is well accepted that I2S sounds better in any system. It is the most direct and least processed connection. There may be some exceptions but they would be rare. I2S is how your streamers and DACs communicate internally on their main board. So anytime you interconnect with a different format you are losing some info thru translation.
Why Lumin does not provide I2S I do not know but they will be left behind soon as that is where things are headed.
On a related topic- DSD will replace PCM format as network bandwidth will accommodate it. Most music is mastered in DSD and then converted to PCM which losses resolution. So PCM will eventually be eliminated. As with I2S interconnect there is a trend towards eliminating translation of data to retain the most accurate signal.