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”!
The test you did Mihalis sounds well thought out and the results exactly as expected.

It is possible for the incoming data stream to impact clock timing but that would be expected in a poorly constructed $50 USB DAC from China not an MSB, not to mention the pro-ISL fiber does not communicate USB.

There is also no guarantee that thyname's high end streamer does not manipulate the bit stream or that what he compared it against manipulated the bit stream and the one he prefers does not. We like magic and so we hold on to the magical explanation when the more likely one is almost always right.

Keep in mind a lot of high end components are intentionally euphonic and their euphonic signature dominates their sound not necessarily their technical competence.
Thyname it’s the select dac with the same interface you describe. Since I am receiving bit perfect signals nothing happening prior to the conversion to fiber matters IMO. If the signal was not bit perfect (msb offers an extensive test) then it could have affected things. 
Getting “bit perfect” signal is not a rocket science. You make it sound like people are all naive and streaming music from some noisy desktop computers. It’s extremely easy with all major streaming protocols, including from Roon via a NUC, Nucleus, Innuos, and so forth.

MSB Select DAC starts at $85,000 I believe. Once you add your ProISL / ProUSB module (that alone is $2,000) and perhaps the second power base, and the upgrade clock, then the price becomes ridiculous.
My test say different. I just got into streaming back in the summer and purchased a Bluesound Node2i. While it was a nice a streamer for the money, I was missing something and I also wanted a server to rip disc to replace my old Sony DVP9000ES. I bought an Innuos Zen mkIII. Yes almost 5x the cost of the Node 2i. I received it this past Monday and hooked it up. My daughter who is now 22 and has started appreciating music but is no audiophile like me, she was sitting on the sofa on her phone. I put on a familiar song and her head perked up as did mine. The Innuos was much fuller, richer in sound. Once you sat in the magic spot you could hear the improved soundstage. We let it play a while and then switched back to the Node since I still had it hooked up to the same Qutest DAC, we both looked at each other and smiled. Even my wife heard a difference. My system is pretty simple. Older pair of B&W CDM9NT being fed by a PrimaLuna EVO300 integrated being fed by a Chord Qutest DAC previously being fed by the Node, now the Innuos Zen.