It depends on the DAC. I own two Holo DACs (Spring 2 KTE and May KTE), and USB from a good source is a better choice with them that I2S. I drive them from small PCs (Up Gateway) running NAA with Intona isolators on the USB path. I’ve used them with I2S from a variety of sources, the USB setup I have now is as good or better.
I agree with Fernando. My May KTE sounds better being fed via USB than it does via I2S…and I have tried a couple of different DDCs…
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I was talking about the PI 4 needing a DDC to sound better as it has fairly limited USB circuitry.
For the case where I went direct I2S from a Mercury 2 into a Denafrips Venus II - the DAC has temperature compensated clocks already and very good power and mainboard components so no DDC would be necessary. Where you have a lower quality DAC the Denafrips Iris DDC gives you the same temperature compensated clock chips as the Venus II and will do a better job at timing the signal.
The reason I pulled the Lumin U1 out of the system was the combination with the Venus II thru USB sounded a bit more processed than the Mercury 2 direct in with I2S. Also USB adds more circuitry to the signal chain as you have to transcode I2S from the streamer into USB send it to the DAC and have the DAC transcode the USB back to I2S for its mainboard.
Generally less is better considering signal degradation. IMHO.