I had forgot that I had actually a 5-cable-method going on; I had a line-out from my poweramp to my DAW. I came to unplug that and.... SILENCE! That noise was gone!!!
Now I can use the 4CM completely. I just need to find a way to get that line-out working with all these. Or maybe when I wish to reamp that signal I will just use a laptop/iPad for that. I still wonder WHY did the noise disappear when I unplugged that line because it was ran to my DAW through a transformer-isolator :S
Nevertheless, now I can reamp my real stack with helix!!! (if only did the audio work stable with El Capitan... the audio driver needs to be rebooted every once in a while...)