[1] I'm not confident it's fair to extrapolate from there that USB is inherently quieter than coaxial.
The noise floor measurement notwithstanding ([2] either number is beyond perception, especially with a signal present), I have found USB to, at best, equal coax in a particular system. [3] I've never heard it outperform coax in any system where a comparison was possible.
1. Agreed. Going on these measurements alone we do not know if USB is inherently quieter than coaxial, all we know for sure is that the output of this particular DAC is cleaner when fed a USB signal than when fed an S/PDIF signal but these measurements do not indicate which of these two possibilities is the cause: A. That the USB signal is actually cleaner/quieter than the S/PDIF or B. That the USB signal is noisier/dirtier than the S/PDIF signal but that this DAC is capable of cleaning-up that noisier USB signal to a higher degree.
2. I wouldn't agree that these numbers are beyond perception, beyond audibility certainly but not necessarily beyond perception. There are no numbers/levels which we cannot perceive, we can perceive differences of ANY magnitude, even when the magnitude of difference is zero (there are no differences whatsoever)!
3. If "either number" is beyond audibility then obviously you've never heard USB outperform S/PDIF, just as obviously, you've never heard USB under-perform S/PDIF either! Of course though, you haven't been comparing systems with this particular DAC, you may have used a much poorer quality DAC but for the noise to reach audible levels it would have to be a particularly crappy (or malfunctioning) DAC.
Further those results are only on HIS system. There are far too many variables, different DACS, different USB transceiver cards, etc. to come to any worthwhile conclusions with those results.
We can of course make some conclusions from those results but whether these conclusions are "worthwhile" to you personally is a different question. We can make at least two conclusions: 1. With an average consumer laptop and generic USB cable this $129 DAC can reduce whatever artefacts (interference/noise) is present at the output of the generic USB cable to levels well below audibility. 2. An audiophile grade USB cable COULD NOT make any audible improvement to this setup.
These two conclusions raise some obvious questions: 1. If a $129 DAC can reduce USB noise/interference from a consumer laptop with a generic USB cable to well below audibility, would you not expect any other similarly priced or more expensive DAC to perform equally as well or better? 2. Even if we assume that an audiophile USB cable performs significantly better than a generic one (despite the fact there is little/no evidence to support this assumption), if you have a DAC which performs audibly worse with a generic USB cable, surely the best solution is not an expensive audiophile cable but just to buy a competently designed DAC (for $129)?
It's entirely possible that these conclusions and resulting questions are not "worthwhile" to you, in fact they might be quite the opposite of "worthwhile" as they contradict a belief/opinion which you obviously don't want contradicted. That doesn't mean they are not "worthwhile" to others or in general though!
You just need a DAC whose designer knows USB signal is not clean.
I would go a step further and suggest that all you need is a DAC designer who knows what USB is. The mistake made by many audiophiles (inspired by marketing BS) is the erroneous notion of some ideal/perfect USB signal which cannot be achieved in practise and therefore using an audiophile cable which improves this imperfect real world USB signal will result in better/quieter performance from the DAC. This notion is erroneous because the USB protocol does not assume an ideal/perfect signal, quite the contrary in fact, it specifically requires an imperfect signal! For example, the USB specs does NOT specify a square wave, it specifies a range of "eye patterns". A USB specified DAC should therefore operate optimally with any USB signal within this USB specified range. Even if an audiophile cable did "improve" this eye pattern it should not make ANY audible difference to the output of the DAC, as a generic USB cable is still transferring a signal within the USB specifications by virtue of the fact that it is a USB compliant cable. If there is an audible difference the DAC is not operating as a USB certified DAC, it is faulty. The USB specs also include a 5v power supply and isolation from EM/RF interference.
In other words, the USB specifications explicitly defines a non-perfect/noisy signal in the first place and this is therefore what a USB specified device MUST deal with. If it can't, if it needs some sort of additional USB signal cleaner (device, cable or whatever) then it is NOT a USB compliant device, it's a non-compliant device, a faulty, malfunctioning and/or falsely advertised device!
All the above is not generally the issue though. Few DACs are so incompetently designed/falsely advertised in the first place and few audiophile cables would "fix" those incompetent designs anyway. The majority (probably vast majority) of the time, the issue is audiophiles perceiving inaudible differences, IE. Issues/biases with their perception rather than anything related to sonic differences.
G