Does your DAC sound better through the S/PDIF or I2S inputs than through USB?

It’s an apples to oranges comparison because the SPDIF connection goes through the U18 that may be providing a better clock, galvanic isolation, etc.  Also may obviously be affected by differences in quality between your SPDIF and USB cables.  The R26 is a good DAC and the Gugnir may just be a sideways move and think you’d need to go to a Yggy to get a meaningful improvement, but at that level you might also consider the Denafrips Pontus 15th as well.  If I was you I’d try this HDMI cable — I use this between my DDC and DAC BTW to good effect — for about 7 bucks (for a 1 foot cable which is what you want to keep the i2S connection as short as possible, which is important) and see if the i2S connection makes a noticeable improvement, and if it does you might consider getting a better HDMI cable from DH Labs, etc. at some point.  Just some thoughts FWIW.

https://www.monoprice.com/product?p_id=24182

My take is there are no universal rules in regard to interfaces, each situation unique. One should research the quality of each interface on both streamer and dac, then try to match the two. Generally we find usb is the de facto interface these days, this based on some faulty assumptions IMO. I2S is the native signal path within dacs so why would we want to detour through a usb board when an I2S interface would be the most direct path. The most often given reason for not providing I2S interface is I2S clock not designed for external connection to dac's internal I2S data path. This is true, reflections on I2S cable can cause some amount of jitter, this is reason to keep I2S/HDMI cable short as possible. So, in considering using I2S interface we much consider some trade offs, the extra amount of jitter using external clock vs dac's internal I2S architecture. Assuming we're using DDC to accomplish this, one has to compare the quality of clock and power supply to that clock in DDC vs dac. Specifically, does the clock and power supply to that clock in DDC with the possible added jitter outperform dac's internal I2S clock and power supply. I'd posit a DDC with quality OXCO clock and power supply is likely to outperform a TXCO or Femto dac clo ck virtually every time, this even with the small amount of added jitter via DDC. And when comparing to usb one should also consider the advantages of discrete runs for data and clock using I2S/HDMI.

So the above assumes dac even provides I2S interface.  Minus that, usb is not a bad interface, many dacs have very nice usb boards and quality streamers also have very nice usb interface. Another consideration with using I2S, interface INTO DDC is of extreme importance as well, this means usb for many, so don't scrimp on usb even if using I2S. Bottom line, best interface highly variable.

@soix 

Good points.

My USB and Coax cables are both very good custom jobs by Igor Kuznetsoff of the NJAS, about the best cables can be. I'm not sure the DAC takes the clock from its SPDIF inputs.

I've tried the DAC with two different DDCs now: the U18 ($500) and the iFi iLink (small, inexpensive). My iLink has some custom modifications by Igor and is powered through a very HQ 5V LPS. The iLink probably doesn't have that great a clock. Yet the R26 sounds much better through it than through the R26's USB input.

So I'm not sure the DAC even takes the clock from SPDIF.

I've also tried the U18 with an HDMI cable, an Audiquest Firebird 0.75 M, their 2nd from top of the line. It's probably compromised by being longer than necessary. In any case it sounds way better through Coax, maybe because Igor's coax cable is better.

The R26 is fantastic in some ways, but it has a quality control problem and it's the second R26 I've owned that seems to be changing sound over time. I've read some stuff on R-2R DACs that indicate they have variable quality because the precision required of the resistors is so demanding and resistors can drift. So my current R26 is brighter than when I first got it... I changed the filter setting to prevent the harshness that it developed. I'm quite sure the harshness wasn't there when I got it, because I was happy with it on one particular filter setting, and over time I started noticing I really didn't like the sound of my system in unmistakable ways. Now it's okay on the NOS setting.

But I'm thinking I want to get away from R-2R DACs.

Yes my DAC sounds better from S/PDIF than USB. 

I have a crazy set-up because it sounds better to me. 

I stream from an Innuos Statement via Ethernet to  a Sonore  OpticalModule  that sends fiber to a Signature Rendu which sends USB  to an Antipodes S20 which sends S/PDIF to a Lampizator Golden Gate upgraded with the JL Audio USB.

While it makes no sense to me that a direct USB connection between the Statement and Golden Gate is inferior to the above, it does make me skeptical about using USB as both Innuos and Lampizator highly tout their USB. 

I2s is the best, followed by anything else, I used Ethernet before i2s, last is usb. My $1000 i2s cable is .75 meters long. A lot of the better dacs are not including usb, while some like Berkeley make a $2500 box to try to make usb sound better. https://www.berkeleyaudiodesign.com/alpha-usb2
 

Most people when asked this question state usb sounds better because they have never tried i2s. Why? because most dacs don’t include it, and most servers/streamers don’t provide it. The better products do provide i2s. Over a couple decades, when you saw the proprietary digital cable going from an sacd player into the preamp/integrated (like what Esorteric es-link provided as an example), was using i2s.