Hello Cyrille Damez.
I'm very, very glad for having your technical answer to my problem. I'm an old engineer who is trying to move to another field, the architectural visualization.
As I infere from your answer, it seems to be a different layers stack working in the Substance plugin for UE4 than the one working in the Marmoset Toolbag plugin/module.
It is being compressed by the substance engine after the texture has been drawn, and before storing it in the graphics card video memory.
Therefore, either it is the UE4 Substance plugin who compresses the in-memory-bitmap before submitting it to UE4 or it is the plugin itself who directly stores the bitmap in the video card memory. I didn't think it works this way.
Marmoset is ... I suppose it does not compress the textures because it aims at maximum quality.
Therefore, in this case it is Marmoset Toolbag -not the Substance module- which is the responsible of compressing the textures and storing them in the video card memory. I thought this is how it worked in UE4 too.
You should be able to switch individual textures' compression on or off in UE4.
I have searched this alternative for long before posting my question in this thread. Haven't found it either.
I'd like very much should you be so kind to clarify the above questions for me (the layering of the engine) due to my incurable tech curiosity. On the other hand, perhaps the more pragmatic and urgent question is about whether is there any workaround. Can I generate high-frequency normals outputs in UE4 with the Substance product?
Thank you.