Found the best solution for my R26 was Gustard C-18 clock output to U18 DDC (set to Ext. Reference). Use good quality coax cable here. Experiment with square wave vs sine wave clock output. Sine wave output is less sensitive to cable impedance (U18 clock input is 50 ohm - make sure to use equivalent impedance cable if using square wave output).
U18 is fed USB from server (in my case Antipodes K30). R26 gets the I2S output from U18 (make sure R26 is set to Int. Ref for clock). I2S cable supplies data and 10M externally referenced clock signal from U18. Once again - a high quality !2S cable is important (Tubulus for me).
Sounds great to me. In this setup I believe the overall quality of the server is of greater importance and contributes more to the sound quality than the dac. I'm feeding the U18 with upsampled PCM (via PGGB - files served from internal SSD on the K30) at 705/768 kHz/32 bit. The R26 is set to NOS mode.
Dave