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”!
Thyname thank you for the more extensive response which I hadn’t seen until now. Going for that bath now! M.
@mihalis I simply shared what I experienced, with that MSB DAC and that same module. Which MSB DAC did you own? I don’t see it under the System for your current gear. So for the ProISL / ProUSB module combo, was it the same you had? That’s what I had.

My subsequent attempt in technicality of the benefits of USB protocol in streaming (or lack thereof) was simply based on my limited technical knowledge. I don’t pretend to be fully able to explain everything I hear.

As for the MSB ProISL / ProISL implementation, yes, the main objective is to eliminate copper, therefore the electrical interference associated with it, by introducing fiber. But it’s not as simple as that. The first device, what they call ProUSB (the dongle that stays outside the DAC, on it’s own) converts USB to their proprietary ISL audio transmission via the fiber. Then the module that goes into the DAC, what they call the ProISL module, is the one that is directly connected to the DAC architecture. Basically copper USB cable to the ProUSB dongle input, then fiber optics cable from the ProUSB dongle output, to the ProISL module (attached to the DAC) input.

So it’s not as simple as fiber, and here you go, all problems solved magically. The conversion itself is perhaps just as important, as in theory can introduce other issues.

Further, many of the non-believers will tell you that any of that is not an issue at all, and any competent Topping China $50 DAC has all that built in, galvanical isolation and all, so totally unnecessary to go through fiber they will tell you. So here is that.
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.