Why are digital streaming equipment manufacturers refusing to answer me?


I have performed double blind tests with the most highly regarded brands of streamers and some hifi switches. None have made any difference to my system on files saved locally. I have asked the following question to the makers of such systems and almost all have responded with marketing nonsense. 
My system uses fiber optic cables. These go all the way to the dac (MSB). Thus no emi or rfi is arriving at the dac. On top of this, MSB allows me to check if I receive bit perfection files or not. I do. 
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. Your signal can’t be improved by any of these expensive pieces of equipment. 
If anyone can help explain why this is incorrect I would greatly appreciate it. Dac makers mostly agree, makers of streamers have told me scientific things such as “our other customers can hear the difference” (after extensive double blind testing has resulted to no difference being perceived) and my favorite “bit perfect doesn’t exist, when you hear our equipment tou forget about electronics and love the music”!
mihalis
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Btw the designer of maybe the best steamer finally confirmed my straw man assumption. Turns out indeed it was correct. The signal is identical. No one can hear any difference.Of course I wouldn’t question that the person who was very offensive for no obvious reason earlier would hear differences from these identical signals from up there at his high horse. M.

@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.
Hi kijanki, thank you for the clarification which I understand. 
My statement was a straw man assumption which I put forward for us to debate. It is “absolute” on purpose, like a math equation that we are trying to prove wrong. Sorry if I wasn’t clear about that and it sounded like I was saying that I know it is correct. Which I don’t.
The revised statement which is now informed by further feedback from two designer experts is that so long as we receive bit perfect signal via fiber optic cable through a converter that is clocked to the dac we get a signal that can’t be improved with expensive streamers. One said “everything before the conversion doesn’t matter at all”.
Some other arguments against this include that one may be introducing electrical noise to the house’s electrical system before the fiber and also that one may be introducing airborne ref. I suspect these are negligible in real life conditions especially as one can move the server to another room.

I repeat this is an assumption for debate although we now have many facts in its favor. And I hope it helps people save money and focus their spending on equipment that matters. 
What I am baffled by is 3-4 responses which were not only rude but also paradoxical: bashing an effort to emulate the scientific method whilst using such science based equipment. Maybe it’s the snake oil they put on their salad. 
All the best and thanks to those who tried to explain and help. M.