Looking for help understanding clock usage in my system

This is a complex clocking configuration question (rather than which clock is better), rooted in the fact that an external 10 MHz clock feeds each device’s PLL, providing a long-term timing reference. The PLL continuously corrects the local oscillator using this reference to improve long-term stability. The external clock therefore serves a different function from the internal audio clocking scheme, which makes it difficult to determine whether a higher-quality clock inside a DDC can truly replace or outperform the clocking role of the LHY external 10 MHz reference.

A well-informed answer ideally comes from someone who has experience with all the components involved. You are the best person to answer your question and help the community by clarifying whether (1) the U26 performs better than the U18, and (2) whether using an external LHY 10M clock across all connected devices results in audible improvements, and in what specific ways those improvements manifest.

@lanx0003 thank you for the reply.

I will try the U26 with and without the external clock and see if I notice improvements.  And, see if the U26 sounds better than the U18.

I like the way my system sounds.  I kept reading that the U26 is noticeably better than the U18, but does that include a U18 with an external clock?  Your suggestion of trying square wave was eye opening.

I found a LNIB U26 on USAM.  I suspect I'll keep it and probably sell the U18.  

I suspect that by using USB into your DAC, the DAC is now in charge of the clocking.

Generally with AES (or coax)  connection from streamer to DAC the streamer is controlling the clocking.  With USB into the DAC the DAC is holding the clocking.  So in your description, I do not know if all that pre DAC clocking is as helpful as using the clock on a DAC that accepts a clock cable.  Of course an external clock may or may not be as good or better than an internal clock, but has the advantages of a separate housing and separate power supply.

@fastfreight To clarify, the OP uses USB into the DDC and I2S from DDC into the DAC, and therefore all clocking duties are performed by the DDC. I think the way it was worded may have been just a little confusing perhaps.

@sls883  According to Gustard, the X30’s external clock input is active only when the DAC is receiving a signal via USB or LAN. For all other inputs (I2S, SPDIF, AES EBU), the X30 has a PLL that reclocks the data using its own internal clocks. The only time the PLL is disabled is when using NOS PCM or direct DSD. If you are not using either NOS mode or direct DSD, you should find the connection between clock and DAC superfluous. Of course, with the X30’s PLL reclocking incoming data, it remains in question that a DDC (and external clock) provides any clocking benefit at all. Gustard’s customer service admits this. In that case, the primary advantage of the DDC would be galvanic USB isolation.

The OCK-2 is a nice unit for the money, and I had the upgraded OCK-2S until recently. It has been pointed out elsewhere that the square wave converter in these clocks is a primitive circuit and not on par with more expensive studio clocks (e.g. Mutec). I see you prefer that setting, but most find it sounding "edgy" as compared to sine wave (just FYI).