What did a good 10 MHz clock do for your Gustard R26?

@audphile1 

While I do recognize that my system might have some kind of "signature" as a whole that is obscuring these components, I've had a very good experience with Igor's mods. I started with just his power conditioning (but all stock components at the time). It made my system more extended on top and more musical by quite a lot. In particular it made things more extended by not brighter. Everything he's done since has built on that. I've carried some of his modified components to other people's systems and they bring similar qualities. Like, a long time ago when I still used CD I brought his modified PS Audio Lambda II to a friend's system and it improved the extension without making the system brighter. I've been in contact with folks in the New Jersey Audiophile Society who use his cables rather than the $5000 cables they could easily afford. He mainly takes moderate priced equipment and brings out the potential - so for instance my headphone amp is a Chinese 12AU7/FET zero-feedback design that comes with a SMPS ($100 new!). He replaced all the parts, added shielding through ERS fabric, added damping, and designed a LPS. Many people in the headphone community praise the concept of 12AY7/FET designs, so it's not surprising how good it is.

With the PS Audio DAC, I'm feeding that by a stock Aurender through Igor's USB cable. Tell you what, I'll replace that with a stock USB cable and test it. Then everything will be stock components except the headphone amp. I'll try it with several headphones through their stock cables. Let's see if it changes the tonal balance or component signature.

@magon actually not a bad idea! I always go back to stock cables to set a baseline for a component’s sound when evaluating it. Aurender includes a very nice usb cable if you still have it, worth a try. Or just use any basic usb and power cord. 
Awaiting your updates. 

Looking at the back panel of the U18 there is a BNC connector marked as an input "1OMCLK".  There is also a BNC output marked "WCK".  If I understand the post, I can use my Afterdark clock with the DDC and then simply use an HDMI connection to the DAC and I will get the benefit of using the external clock with the DAC as well.  Is this correct? 

Yes, correct - Just make sure the DDC is set to External Clock and the DAC clock reference is set to Internal when using the I2S input.

Thank you, dspringham 

@audphile1 Whoa... went to an iFi USB cable (maybe $150 when I bought it 7 years ago) and the brightness in the PS Audio is gone! The strangeness in the bass is gone. It's still a bit bass-light compared to the two Gustards I've had in my possession, but it's much closer to something balanced. It's also less exciting, less microdynamic, less resolution.... but it suggests that I could look for a USB cable with all that resolution, and also the correct tonal balance to boot. Or perhaps drive a D-D convertor through my original USB cable and then drive the PS Audio via coax or something else. 

Huh....