@maprik There seems to be a philosophical difference between manufacturers of digital gear between USB and I²S as the connection of choice. Either is (usually) better than Coax, TOSLink or AES/EBU - though Aurender recommend their Coax for some streamers.
Your IRIS box tidies up the S/PDIF, reclocks it and outputs the I²S streams (data, bit clock and word clock), the DAC then uses the clock signal from the IRIS, which has been transported over the HDMI cable, Hence the critical nature of the cable.
Esoteric use a proprietary current based flavor of I²S, ES-Link, for their Grandioso separates. At $100k or so. I guess that they decided that the best bang for the buck in the $15k to $30k range was asynchronous USB with a precision clock and a FIFO buffer.
There are (heated) discussions of the merits and problems with USB and I²S on this Roon forum https://community.roonlabs.com/t/usb-and-i2s-which-is-better-for-connecting-dacs/274372/8
Denafrips have chosen I²S as their "flagship" interconnection, and, yes, the cable quality is critical. The routing of that cable may matter also as it should be away from EMI noise.
This is the sort of discussion that helps to make the equipment side of this hobby fun!

