Gustard A26

@magon - I have not heard the A26 but if you live in the US you can buy one on Amazon and return it to Whole Foods so it's pretty low risk.  I do have an X26 Pro.

I'm looking at your connections with the DDC.  I think I read on this forum that if you connect your clock to the DDC and then connect the DDC to the DAC via HDMI, the DAC "sees" the clock as well .  Might be worth a try.

Maybe the A26 would be a solution. I’m using a 10 MHz clock (the Gustard C16) and a good clock cable, along with the U18 DDC to feed the DAC through a very good Coax cable.

Any cable would ruin your 10 Mhz clock. The external clock phenomenon that got sold to you is snake oil.

All the greatest clocks on earth should be inside your dac box right before the conversion occurred.

I find the R26 has some remarkable qualities in terms of microdynamics, musicality (musical details), but poor quality control and among the 4 R26’s I listened to, all had (different) tonal balance problems.

You bought four R26 dacs, A/B/C/D compared them back to back, and heard "tonal balance" problems on each unit??? This is getting a li’l hard to believe.

I've tried a few Gustard DACs over the years, including the A26 recently. They all seemed nice but had some sort of major bug or malfunction to where each one got returned. One was DOA and its replacement had major issues too. I don't know if I'm just unlucky or what. 

Very easy to believe there are inconsistencies between same model DACs…i.e. R26. These are, after all, made in China sold on amazon cheap low quality garbage stuffed with attractive features.
What is hard to believe though is how persistent one can be in a desire to obtain an imaginary perfect copy. After trying several and concluding they’re not good, just move on dude. You think you’re going to hit a literal and stumble upon one that accuse sounds like MSB Reference? If Denafrips made a DAC with streamer, MQA, wifi, display, etc. and it sounded good, it would have a starting price of $6,000. Forget this junk. Get something that’s made to high standards.