Just about to pull the trigger on Pontus 15th ...

@lanx0003 So, I decided to try the direct USB connection between my streamer and dac eliminating the DDC.  It still sounds really good, but I do notice a little bit less detail and clarity.  Like you said, that could be the DDC, or it could be that my dac is optimized for I2S.  I have no way to test it because my Innuos only has USB outputs.  

Anyway, I'll leave the DDC in the chain.  

Thank you for the feedback, @sls883.  Have you also used 10Mhz clock into U18?

OK. I read your question again. You were asking about the clock into the ddc.  I was commenting on the clock into the dac. 

I hadn't really tried switching between the external and internal clocks on the ddc. So, I tried that tonight. I do think the external clock helps here. Seems like a little more clarity and punch.  

I don't exactly understand how these clocks work to be honest....

It makes sense.  My understanding is that, in the chain

Innuos (usb) -> U18 (i2s) -> X30 that you have,

U18 is doing the re-clocking of usb signal.  If using external master clock in U18, it will help clean the output from U18 going into X30.  The cleaner the output from the upstream DDC, the better (less jitter and phase noise) the signal will go into DAC.  So in this case it is better off to use master clock in U18.