*Update*

I finally obtained a cable, which allows me to connect the Eventide with the system as I did it back in the days, where no noises could be heard at all (at least not that I remember).

Which means basically…

Eventide -> Mixer -> Headphones.

In the end, there's no further amplification – just the Eventide, but hey, the noises are still there.

That way, I have to admit defeat and the machine needs servicing, which is… oh god… noooooooooooo!

At least the speakers aren't at fault.