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

Yes, I have.  To be honest, I don't hear any difference, but my dac has a pretty good clock built in. 

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. 

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.