I have used both optical and coax (75 ohm cable), both sound great with my Yggdrasil. Since all 2i outputs are on at the same time (my integrated has no dac) I can run RCA’s to my integrated and coax to my Yggdrasil and A/B the Dac’s, prefer the Yggy, however the BS 2i is a good DAC. Lots of options enjoy. Also the Yggy doesn’t do MQA, nor does the Chord I believe, so with my set up I can compare MQA via the 2i direct to my integrated to MQA through the Yggy, with a partial unfolding, interesting option/comparison.
