Why are digital streaming equipment manufacturers refusing to answer me?

Hey @mihalis I had the same MSB DAC, but the lowest model, the Discrete with the same ProISL / ProUSB module combo like you. At that time, there was an audible difference to my ears when feeding that DAC via USB from a NUC, then Nucleus, vs. a top server ‘ streamer I had then, the Innuos Statement. That’s what I heard, with my ears, my equipment, my room. That’s all I needed to know, and be happy, enjoying my music. I did not have to worry why, although I intuitively can figure out the reason, based on what the Innuos Statement has / does.

All what matters is we experiment for ourselves. Sometimes new things work, sometimes they don’t. We then move one to something that works, or just stay with what we have / own. It’s pretty simple. 

@mihalis  yes, asynchronous USB receives just data and uses own independent clock for D/A conversion.  Still, any electrical connection can inject noise and alter this clock timing.  As for optical connection - it helps, but it is not a panacea.  Bits are still bits but timing of arrival can be altered.  Any system noise can make transition (including light intensity) "jagged" varying exact moment in time of level change recognition (threshold).  I don't know how to explain it better, but let's try this - Imagine in slow motion filing your tub with water.  It supposed to stop when crossing certain fill level, but water have waves (noise) and every time you repeat it - it stops a little bit sooner or later. That is time jitter.

Perhaps you know all this, but imagine that you receive 1 kHz pure sinewave.  Big electrical noise added to signal is causing different moment of level recognition and stream to "vibrate" in time.  When words fed to D/A converter vary in time then it will result in creation of additional signal - sideband frequencies.  When 1kHz stream delivery vary in time by 20 milliseconds (50Hz) the output of D/A converter will produce 1000Hz, 950Hz and 1050Hz - three frequencies instead of one (and many more at lower levels).  Amplitude of these additional signals will depend on range of time vibration, but it is very low.  It is still very audible, since not harmonically related to root frequency.  Many noise frequencies and many offended frequencies (music) results in added noise - less audible for random jitter (uncorrelated) than jitter induced by particular frequency (correlated).

As for a USB streamer, I wanted to clarify the “clocking” part on this discussion, based on what I researched and experimented with back then (I no longer use USB now):

Yes, the AUDIO SIGNAL on the USB streamer is “clocked” by the DAC. No question. A good USB streamer reclock the USB signal, not to be confused with the clocking for the audio signal. It is reclocking the USB commnunication between the source and the DAC, meaning it does not work at the audio level but at the USB protocol level.

I don't think people realize that the USB signal does not get reclocked by the DAC. What they reclock on the DAC is the audio signal that is transported via USB. The USB communication components themselves (I.e in a USB streamer) are affecting the performance of the DAC. This has also independent of the galvanic isolation of the DAC, which on its own is a separate, independent issue.

I hope this makes sense @mihalis . Needless to say, theory is just that, theory, and not a substitute for practice.

I assume you meant prousb and not usb? Did you run bit perfect tests to ensure you were comparing apples to apples? 
It is encouraging to hear you got improvement and that it was pretty simple. Would you mind elaborating how you can intuitively figure out the reason based on what the innuos statement has / does? 

I don't think people realize that the USB signal does not get reclocked by the DAC. What they reclock on the DAC is the audio signal that is transported via USB.

In async USB DAC receives "frames" of data usually at 1kHz rate and feeds them into buffer.  It signals back upon buffer under/overflow to adjust size of incoming frames.  It is data coming in - not "audio signal".