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

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. 

Also, @sls883, does your external clock have multiple outputs, like the LHY OCK-2? If it does, try sending the square wave signal to both the U18 and X30 and see what kind of difference it makes.

@lanx0003   I have a LHY OCK-2.  I'll try that some time. 

@lanx0003 Maybe my hearing is just better early in the day, but changing to square wave connections to both the ddc and dac is sounding really good.  A hair more clarity and better separation of instruments and harmonized vocals.  Bass notes seem better defined and punchier.  I'll listen more tonight, but I think I'll leave it this way.  Thank you for the suggestion! 

@sls883 

That’s fantastic. I believe Gustard gear works with either waveform, but according to their website, a sine wave connection is actually recommended—and the chart reflects that. You might want to try and compare both.

The 50-ohm impedance matches, right?

This suggests that the LHY 10MHz master OCXO is quite good, elevating the performance of already state-of-the-art gear. I’m tempted!