Looks like USB is your obvious choice from Aurender to Iris. Iris and Pontus both support AES/EBU and I2S. The problem with I2S is there isn't a good standard and few great cables. I would lean toward AES/EBU. I'm using an audiosensibility silver signature cable and it is spectacular. Avoid optical and coax digital.
all digital cables should be either silver or silver plated. That is what my ears tell me and I think it handles the high data density better. also, forget everything you've heard about silver wrt analog cables.
Jerry