The purpose and reasoning for DDC/I2S explained well from above posts. I'll add its not only clock topology that's important, implementation of these clocks extremely important, this means power supplies must be optimal. What this means is a well implemented TXCO or Femto clock could outperform a poorly implemented OXCO clock. I should also add the clocks themselves can vary in quality.
Quality of streamer may or may not make a difference, its all about interface quality/implementation\. Don't automatically assume any or all interfaces within any streamer is optimized to the highest level. A quality DDC unit that optimizes both input and output interfaces, and has optimal clock and clock implementation may offer superior sound quality vs streamer. And then we have the possible inherent advantage of shorter signal path within dac when using I2S interface (dacs convert any input to I2S), one avoids using other input detour via ddc with I2S out. And then we have to consider how well I2S implemented DDC vs. dac, is the dac internal I2S equal, superior, inferior to DDC I2S? All in all there are so many variables at work here, main one is how does DDC I2S compare to the particular dac you're using or contemplating purchasing. I chose the Denafrips Gaia for it's greater potential in providing superior I2S vs most any dacs internal I2S.
