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
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.
I’ve auditioned Node2i, raspberry pi4, SimAudio, Bel Canto, Innuos, Lumin, Auralic and I haven’t noticed a dimes worth of difference between them when used with Roon. I will say I have noticed improved sound through a couple  when using their APP but I attribute than to differences in the software.
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.
Its incorrect because its wrong. Your whole premise is false................Not only wires, I can tell the difference between lots of things- wire on the floor vs elevated, wire going one way vs another, warmed up vs cold, on springs vs cones, on and on. And on.
LOL.  Thanks for the facts mihalis and welcher.  Great discussion.