I was going to mention what you just summarized in my next post.
In the case of your Dave, it buffers all digital inputs including optical spdif (into RAM), then reclocks the buffered spdif before sending it on through the conversion process, which most DACs don't do AFAIK. You'll notice the clocks inside a typical DAC are implemented by means of the USB PCB (next to the USB chip itself), so I'm not sure how you'd access them for other digital sources. Again, your Dave is an exception to this. Most of the time, you are at the mercy of the clocks in the host (or bridge converter) that is sending the DAC its signal. In general, the spdif protocol is not asynchronous, so once it leaves the host (ie MAC) that's as good as it's going to get. This speaks to jitter. By materials used in optical cables, there is inherent galvanic isolation, so no "noise" can enter the DAC.
The issue you have already pointed out with USB is the "noise" entering in on both the power conductors and data conductors AND ground pin plugged into the DAC. Jitter as well if the USB chip and clocks aren't of sufficient quality.
I will say right of the bat that I can't tell jitter apart from noise. And usually the effect is subtle to me. So much so, that if I didn't have a highly resolving system, I probably couldn't tell the differences between input methods! Whether there is noise or jitter, the results seem to be the same for me: less precise imaging and instrument separation, less authoritative transients, perhaps some muddy treble splashiness, and an overly hollow and loose sound to the music presentation. It doesn't sound "bad", but it just isn't as "in focus" and "right sounding". When you eliminate or greatly reduce these artifacts, it just relaxes and sounds "right" and not forced. It's hard to put it into words
Having said all that - and presenting this next section with the caveat that this is for my DAC (Metrum Amethyst).......round one goes to the iUSB 3.0 reclocker/noise canceller toy from iFi. It definitely helps and is an easily audible improvement over the tosslink on the Mac Mini. The optical wasn't nearly as bad as I was expecting, but it's noticeably not as "in focus" as the iUSB. Surprisingly, it only sounded slightly worse than a direct USB connection! And so much so, that if I wasn't able to switch quickly, I'd never pass a blind test. So the tosslink is not bad at all, hanging in there against the decent HiFace2 XMOS 8 core USB interface of the Metrum.
Here is the ranking so far for my Mac Mini 2012 late and my Metrum Amethyst:
1. iFi nano iUSB3.0 as a bridge between Mac and DAC
2. USB direct connection
3. Optical Spdif/Tosslink direct connection
stay tuned for updated results with Schiit Eitr. Gonna be a while because I'm out of town most of next week and I want to allow at least a day or so of "settle in" for the new Eitr bridge.