I tried it according to the instruction above. My experience differs a bit.
Sitting in very silent environment. I set low gain on Cube (however same result in high gain). Choose music - Mozart piano sonatas HiRes 96/24. Set decent volume. Concentrate if I hear hiss. Difficult to judge, the music is beautiful. Pause music. Then I can hear some weak hiss, not much louder than the hiss I hear when the iem is in my ear, but not connected to anything. Weak, but audible. For 3 seconds. Then the hiss is gone, no difference if I plug or unplug the iem. I can also hear the same level of hiss when I concentrate on it, where there a silence period in the music.
I have not even noticed the hiss until this explicit experiment. I can imagine there are people with more sensive ears on high frequencies, who experiece this differently.
I will continue enjoy music, not concentrate on the existence of hiss
.