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

No

FWIW, a friend of mine has a tricked-out custom-built server with JCAT USB card, and a Chord Dave.  He says his Berkeley Alpha USB in between is a huge improvement over straight USB, even from the high-end card.

Personally I've found that while USB into my Qutest isn't bad, the coax signals from an IanCanada or Pi2AES clock transport are much better.

Hi audio friends. I found in my system ( Antipodes K50 G3 connected to Payback Design Dream DAC MPD-8 Gen2) it depended on the quality of the cable on what sounded best. In blind audio tests with an audiophile buddy of mine who did the cable swapping are my findings. This of course is all system dependent. BTW speakers are Gauder RC7D mk2 which ultra transparent and unforgiving if you feed a bright signal. 
 

Over years of critical listening and countless cable comparisons — USB, Ethernet, and otherwise — I’ve rarely come across a digital connection that transforms the entire character of a system quite like the Acrolink 7N-DA6300 solid core AES cable did between my Antipodes K50 G3 and Playback Designs MPD-8 Gen2 DAC.

From the moment it was installed, the system bloomed with an astonishing three-dimensional soundstage, delivering an organic, utterly non-fatiguing presentation. Instruments and voices locked into place with uncanny realism — each performer rendered with startling depth and presence, not just across a wide stereo spread but in a layered sound field that felt alive. Compared to high-end USB and Ethernet alternatives, the Acrolink AES wasn’t just different — it was in a different league when it came to coherence, spatial stability, and emotional connection.

Intrigued, I later added the Playback Designs PLINK reclocker, using the USB cable supplied by PBD. To my ears, this moved performance yet another step forward. While the tonal balance and spatial attributes remained remarkably close to the AES presentation, the bass gained newfound solidity — not in quantity, but in density and definition. There was also an undeniable improvement in system silence and flow, likely thanks to the galvanic isolation introduced between server and DAC.

Now, six months later, I haven’t felt a single urge to tweak or upgrade anything. The synergy between the Antipodes, the Acrolink AES, and the PLINK-fed MPD-8 Gen2 delivers a level of musical truth I once thought could only be approached in analog. It’s not just technically excellent — it’s emotionally immersive and deeply satisfying.

Hope this helps the discussion. Cheers Mark

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.