I realize you are located in Europe and so you have a different wiring scheme.
But I would also assume that you use 3 wires to connect power to the Mojo amp and the rest of your audio gear.
Now if this isn't the case and you use only 2 wires to connect to your ac power, then this may not work.
BUT if you run a separate ground wire to a source of outside ground (like a cold water pipe etc.) this is the 'source' of the ground used in the system.
As a TEST try unconnecting the ground connection at the Mojo, and see if the hum goes away.
IOW you may be grounded twice, which can cause hum due to a ground loop.
BE AWARE that by lifting the ground on the chassis, it MAY become a source of a shock by touching the chassis.
If this happens pull the plug out and re-connect the ground.
And if you are adventurous (and feel confident to do this) you can try this on other equipment in the audio chain, but be aware the same caution applies about being a potential shock hazard.
Now if it makes you uncomfortable in trying this, then don't and get an electrician or somebody you know who is 'handy' with electricity to help you with this.
But ground loops are usually solvable
Does this answer your question?
Ahhhh, I used a non-grounded wall socket for my Mjolnir and now it's quiet again
Thanks for pointing me in the right direction!!