Digital Audio Output Differences? Is USB Audio Really The Top-notch?

You are asking a seemingly simple question in a complex field.

yes, USB >> SPDIF (or its kissin' cousin, AES) - but only if its is implemented correctly.  Or if the SPDIF is crummy (usually true) or....

So while i would begin with USB, you now need to make sure you have clean power, clean (isolated) ground and a DAC with a good clock since it is not the only thing with a meaningful impact on jitter.  Most modern high end DAC do pay attention to both.  How good they are of course varies.

If cables sounds different, there are serious problems.  Some really are that bad, but its usually something else.  Note that almost every "coax" SPDIF cable on the planet has an improper termination - RCA plug are almost never 75 ohm characteristic impedance.....and it just gets better :-)

I have a RME adi2 DAC , it’s a great sounding DAC to be sure....   I use all three inputs and honestly I can’t tell the difference between my coax and the USB.  It sounds so good from all three , it’s like splitting hairs.   

I2s is the best connection in my limited digital experience but finding equipment from different manufacturers that will have the same I2s configuration is a challenge. I don't stream but tried connecting the ps audio pwt to a gustard x26 pro to play my CDs with zero success. Now I switched to the Jay's cdt2 mk3 and the I2s connection to the gustard is fully operational. The sound is frankly so close to my analog rig now, it's hard to tell the difference. And I have twice the dollar amount invested in my analog rig vs. the digital one. I am completely sold on I2s when feasible. And I also want to take this opportunity to thank the digital audiogon community who steered me in the path of my digital journey. Without it I would still be mainly analog. And CDs are so much less expensive. Today I ordered $120 of CDs on discogs including shipping and I am getting over 10 CDs for it, about a third of what vinyl would have cost me and some titles don't even come on vinyl. This is a huge step forward for me. Sorry for getting off track here but just got through an unreal quality CD listening session tonight and felt like sharing.

USB is the worst connection you can use, even if you buy into all the gimmicks/tweaks you can buy to try to make it better.

Ethernet and i2s are much better, but only your better dacs include them and they are usually more expensive