My configuration is as follows:
NUC+Ubuntu+Core -Ethernet-> Allo USBridge+DietPi -USB-> SU-1 -I2S-> Holo Spring KTE 3
Originally I had Ethernet-> microRendu+LPS-1-> USB -> Holo Spring KTE 3, but then I read very careful reviews that reported that I2S was a significantly better input for this DAC, so I got an SU-1, and indeed the improvement was noticeable. That also allowed me to replace the more expensive microRendu+LPS-1 combo with the much cheaper USBridge without loss of quality. Since then, I read that the Spring KTE 3 now comes with an upgraded USB receiver that reduces the gap between USB and I2S.
There are two possible reasons for the difference between (old) USB and I2S. First, with I2S, the SU-1 clocks are used to time the digital signal instead of the DAC’s internal ones. Second, USB receivers are a common source of electrical noise leakage into the DAC’s delicate digital-to-analog conversion circuitry. Tim at Kitsune posted somewhere (can’t remember where) a recent message detailing the various improvements in this area for the current Spring KTE-3, he’d be the right person to ask whether an SU-1 would be as worthy an accessory now as it was for the original Spring KTE 3.