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

I started using SPDIF converters about 14 years ago, first with optical cables, then with digital coax. Digital coax sounded significantly better than optical or USB straight in from the delta-sigma DACs I had at that time.

Six DACs later I'm still using either a SPDIF converter (main office system) or a DDC (side system/headphone only). Both send signal through quality digital coax or AES/EBU cables (I can't hear any difference between them). All my DACs for the past 6-8 years have been multibit (one) or NOS (5).

Out of all the DACs I've had here, just one sounded equally good via USB straight incompared to digital coax: the humble but pretty nice delta-sigma by Peachtree Audio, the DAC iTx. That's probably because they describe their USB implementation as galvanically isolated (something either SPDIF converters or DDCs do, as well).

@soix great points. In my previous config (Pontus II 12th DAC/Iris 12 DDC), utilizing i2s and USB made sense because the DDC clock was better than both my dac and streamer. I've since upgraded to the Denafrips Venus 15th w/dual OCXO and the Hifi Rose RS130 with OCXO; both have great USB implementations, I see no need for a DDC or i2s. I'm connected USB from the transport to my DAC using a Audioquest Diamond USB and its quite amazing. 

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.

@sns My streamer/server is an Aurender N100H, which offers only USB output, but it’s a much better USB output than the previous server I was using, a simple Intel NUC. I think it’s probably a very well implemented USB output port.

When I use S/PDIF, I’m plugging the USB cable into a Gustard U18 DDC, which has only USB as input. I’m sure that the quality of the Aurender also helps the U18 function better.

My Gustard R26 sounds better, as I said in my OP, through the U18 and SPDIF (coax) inputs. It sounds pretty lame through USB, even when it's the Aurender driving the USB. So I'm guessing it's a poor USB implementation in the R26.

@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.