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

Yep I’m talking about R26. If you have a second unit that doesn’t sound right then you should be done. 
 

there’s a lot of hype going on around the LAiV Harmony DAC. May be try that? Seems to be receiving lots of praise from the reviewers. 

@magon yeah maybe cut bait and run if you're not happy, return the clock and sell the R26, the aftermarket price for them is pretty decent.

I've owned a Denafrips Terminator II (sold it to fund a Lampizator Baltic V4) and would recommend the brand, good sonically, good rep, excellent after sales service and warranty support, seems the new Pontus is a sweet spot in the lineup but you will also definitely need one of their DDCs to fully exploit its capabilities.

Another good DAC I own is the Weiss 204, you won't need a DDC or clock with it, lots of reviews and forum posts about it.

@magon so by now you’ve tried 3 dacs. Gustard R26, your modified Gustard and the PS Audio DSD MkI. Just to summarize…R26 is dark and dull, the PS Audio is too bright, snd the modified Gustard is just right?

This begs the question - have you tuned your headphone and speaker system (s) to the sound of modified Gustard so much that the other two DACs don’t sound right? Both R26 units sounded broken. And I’m most surprised about your assessment of PS Audio DSD. That DAC is anything but bright. So my suggestion based on this DAC shootout you have conducted, is to take a close look at your system, room and the associated equipment as I suspect you have created a synergy for that particular modified DAC and now nothing else can integrate. Is it possible?

@audphile1 I suppose it's possible, but if my system has a characteristic tonal balance, why would it be too dark on one DAC and too bright on another? 

But curiously enough, a lot of my system is modified or custom by the same guy who modified the Gustard x20pro. He did my power conditioning (which is based on parallel capacitance concentrated on my system circuit but also distributed throughout the house), my power cords, interconnects, headphone, and USB cable, and modified my headphone amp. Also speaker cable. My Aurender N100 is stock and I've found that greatly improves any DAC it's feeding compared to my previous mini PC. Obviously I really like the sonic character of Igor's work - very extended on top without being bright, deep musical bass, great dynamics (micro and macro), musical details, etc. 

My speakers are stock - Spendor S3/5, and so is my B&K ST120 amp (from the 70's - great amp - mine is excellent condition and low use). 

As far as what parts of my system affect tonal balance, the headphones are the obvious culprit. I have obtained loans or second hand purchases of several headphones and more will be here soon. I really want to give this a chance before I return the R26. 

What I haven't mentioned is that the modified X20Pro stimulates some kind of resonance in my left ear that has something to do with my tinnitus. Certain pitches sound like they've hit a high-Q resonance. Very odd. I'm going to see an ENT about it. For some reason the modified Gustard stimulates the resonance like crazy, which became more obvious when feeding it from the Aurender, while the modified iFi iDSD doesn't stimulate the resonance at all. And the R26, only a little bit. I want to keep the Aurender as it's so promising, and Igor agreed to trade the X20Pro for future work. So my goal is to match the Aurender with some other DAC that doesn't stimulate the resonance. Oh, the PS Audio doesn't stimulate the resonance.

Thanks for your feedback. I'm sorry I made it so complicated. But I have patience. It's been fun to try a lot of stuff - very educational for my ear. 

@magon interesting. So…couple of things…

I’m also using Aurender, but N200, to feed my PS Audio DSD MkI. I have never experienced any brightness with it. The bass is prodigious. The vocals are velvety smooth. So there’s something there in the associated equipment in your system that doesn’t match. Could be impedance mismatch between B&K amp and PS Audio DSD (not sure if you use preamp). 
As to modified gear - I’ve had bad experience with a modified amp. I loved it in its stock form but it was horrible after mods. It caused tremendous fatigue and I couldn’t listen to my system. That was it for me as far as mods are concerned. Keep in mind that a manufacturer will voice their product to be as neutral as possible. Changing parts, even for better quality ones, doesn’t always result in better sound. In addition, selling a modified component becomes more difficult as the sound signature was altered and the potential buyers understand it. It becomes unpredictable and no one can anticipate what it will sound like. Same applies to cables - the DIYers of the world don’t have the means and the equipment to come up with a cable that would work with wide range of components - they usually tune it to their personal preference using the components available to them at the time. 
I wish you luck in your quest for a better dac and have that tinnitus checked out for sure.