Which PC output is best? Coaxial, optical, or USB?

Im curious of this also

I see all the DAC's with PSDIF and COAX input, and with the async, increased bandwidth of USB, why the need for all the additional inputs?

if it for multiple source units or something or is there a discernable SQ difference

IMO most people condemn the use of USB as it's powered by the computer's onboard PSU and thus, will be prone to more noise and generally degenerate the audio signal, however, theoretically a well implemented USB connection can actually be quite good. As for actual SQ differences, i find that the optical out from the my Mac does slightly better than the USB, it is a very small difference that i found only through excessive ABing though, as usual, YMMV.

Quote:

Originally Posted by Timestretch 

I can't personally hear any difference between the different inputs of my Benchmark DAC1, but I was wondering if perhaps there was some consensus on digital audio connection hierarchy? I know I've heard before that optical might be more prone to jitter than the other connections, and the other connections might be more prone to grounding issues, but I have no idea, really.

There is some truth in it.
Optical by design provides a complete galvanic isolation.
The circuits are a bit slow so the jitter level is higher compared with a well-designed SPDIF over coax (electrical).
The moment you make an electrical connection you run the risk that unwanted signals enter the DAC and disturbs the DA.
SPDIF, be it coax or Toslink is a uni-directional protocol. The only thing the receiver can do is to lock on the incoming signal. If it is jittery, this will affect the DA conversion.
This applies to USB in adaptive mode as well.
In case of asynchronous USB the DAC controls the data send by the PC (bi-directional). This design allows for zero input jitter as the DAC now can run with a fixed clock.
However if this clock is of low quality, the DA still will be jittery.

In practice a ranking makes no sense as the results depends very much on the implementation.
What if you combine async USB with opto-couplers?
Yes you avoid stray signals entering the DAC. The opto-couplers might induce some input jitter.
Etc. etc.

In practice one better look for measurements than design principles.