While I totally support the asynchronous USB argument, I find it interesting that one of the defining features of the "PRO" edition of the Singxer SDA-6 is that it has a Crystek 957 as a USB clock
This is a different situation. The DAC is the recipient device and the Crystek 957 clocks inside the SDA-6 are used to control the DAC itself and therefore have a direct impact on performance. This is not the same as a clock on a USB output.
The clocks in a DAC absolutely matter.
Other USB modules like IanCanada's and JLSound's also support external master clock input
There are plenty of products in audio that have various features and design aspects. But inclusion of something in some products does not mean it's actually beneficial to do.
USB *does* need a clock (48 MHz for full-speed USB 2.0), separate from the "DAC clock" we normally talk about
USB does run at a speed of 48Mhz, but the point is that this clock is not used at all for the actual audio conversion.
Everything is put into a buffer, and then converted using the DAC's internal clocks (such as the Crystek 957s in the aforementioned SDA-6). So long as this clock is not so terrible that the devices are unable to communicate properly or there is a buffer underrun (buffer becoming empty due to not being fed info fast enough) or a buffer overrun (buffer becoming full), it does not have an effect on DAC performance. (You'll know if you have either of these issues as the connection will either fail completely or you'll get very obvious clicking/popping sounds.
If this were the case, then a huge amount of modern USB or network based devices and technologies simply wouldn't work.
but whether the quality of the USB clock is audible after buffering?
As mentioned, if this were the case, it would completely defy our understanding and design of many modern communications technologies and they simply would not work.
Showing a difference in jitter due to different SPDIF, I2S or other synchronous protocol sources is easy. In fact it's pretty easy to even show (very small) differences due to different cables.
But with USB, no evidence has ever been shown that the clock of the USB controller in the host device could have an impact on jitter of the converter. Nor any explanation as to how that could be the case.
(UAC1.0 is a different discussion but basically nothing uses that nowadays so it's a bit of a moot point)