Why are digital streaming equipment manufacturers refusing to answer me?

@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).

So I claim that: if your dac receives a bit perfect signal and it is connected via fiber optic, anything prior to the conversion to fiber optic (streamers, switches, their power supplies, cables etc) make absolutely no difference.

It is simply not true.  Fiber optics transmitters are slow - about 10x slower than coax drivers.  System electrical noise makes those light transitions noisy (jagged), that in effect results on receiving end as a variation in the moment of level recognition (threshold) - a timing jitter.  This jitter translates pretty much to noise added to music.  That's why most often coax works better than Toslink.  You might not hear the difference because your DAC has very good jitter rejection, because your system has limited resolution, or simply because your hearing is not that good  (mine is getting worse).  Whatever the reason is - blanket statements are never useful.

Audio2design, You're right - that's probably what he was talking about, otherwise it doesn't make any sense.  He says "My system uses fiber optic cables. These go all the way to the dac", hoping that optical connection, shielding DAC from electrical noise, makes incoming music always the same.  It is not why it is the same.  Streamed music doesn't come in real time.  It is just plain data that is pre-buffered.  As long as it is the same data in the same format everything depends on the hardware that creates S/Pdif stream/timing (and I assumed he uses different hardware for different providers).  What makes different streaming providers sound the same is the fact that they send data only. Asking them why it sounds the same (with the same hardware and the DAC) is like asking different bookstores why the same book looks the same.

It should sound the same, assuming same internet provider and hardware, same data, same data format, same streaming hardware, same DAC etc.  "Should", because there might be something else that I'm forgetting.

I looked at Toshiba Toslink drivers.  There are some, perhaps older generation, that go only to about 5 Mbit/s, but they have newer drivers that go over 100Mbit/s.  Such Toslink devices should suffer less from the system noise.  Jitter induced by slow transition time and noise at the receiver's (DAC) end would be reduced as well.

@mihalis I called your post a "blanket statement" because you stated that nothing makes a difference when your DAC is connected thru fiberoptics etc.   There is a big difference between saying "I cannot hear the difference" and "Nobody can hear the difference".  There are many factors involved, including receiving hardware and the fact that your DAC is connected by fiberoptic is not changing anything.  I can only agree that with exactly the same data received in exactly the same format in exactly the same hardware to the same DAC - sound should be the same between providers.

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".

Since I am receiving bit perfect signals nothing happening prior to the conversion to fiber matters IMO.

mihalis, that's the beauty of it.  With my WiFi computer noise, speed, amount of RAM etc. don't matter, but there is hardware prior to fiber and it does matter.  As long as you use the same hardware with different streaming providers result should be the same.