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

@magon Presume that very nice cable, like silver for my digital cables, use AQ Diamond for my usb cable. The one thing I'd say about using HDMI vs I2S cable is one may be paying for unneeded runs of wire with the HDMI, silver is pretty pricey.

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.

The salient point using I2S vs all the others is the ability to use a better clock via a DDC vs dac's internal clock. With superior clocking presentation can be greatly affected. Proper clocking means data packets more correctly timed which results in a much more analog like presentation. If you investigate most dac's internal clock you'll generally find Femto clocks with rather pedestrian power supplies to those clocks, one can do better in this case. Not sure I'd want to purchase any dac devoid of I2S input, minimally I'd want to compare my Denafrips Gaia with Tubulus Ximius I2S cable to the dac's internal I2S architecture and/or it's usb implementation. The only higher level dac I know of that incorporates I2S is the Mola Mola Tambaqui.

Regardless of input used on dac, I2S clock and power supply to that clock is the last clock seen prior to analog conversion. One is just adding more clocks using other inputs, how these clocks interact with I2S dac internal clock is variable. Employing highest quality clocks and power supply to those clocks throughout entire streaming chain extremely important for presentation. Presentation means timing, getting timing right means more analog like presentation. High end vinyl setups are the reference for this, listen to this and you'll get an idea of goal to reach for.

No doubt quality of interface in streamer is critical in determining which input on dac is preferred. Many streamers use plain jane outputs on motherboards, this not optimal, reason why usb interface/output like the JCAT and Berkeley provide higher sound quality. And then there are the proprietary interfaces such as Auralic and Taiko provide which may also be a game changer. And then we have the cables, need level playing field when comparing interfaces. Galvanic isolation is the bare minimum for a quality interface, clocks, power supplies to those clocks critical, and then with the outboard devices such as DDC, they need to optimize both input and output. In the end entire chain of interfaces has to be considered in evaluating which is best in any particular situation.

@magon It seems reasonable to conclude R26 usb implementation not  optimal. I'd suggest you need better DDC in order for best optimization of I2S. Jitter specs for DDC don't tell the whole story. I had a Singxer SU6 some years ago, wasn't all that impressed  with I2S at the time, Gaia DDC  with really nice Tubulus I2S  cable makes all the difference. I've haven't used SPDIF as a digital input in many years.