Cutting the ground goes against safety, don't do it!
If even you do, like tying the chassis together, as I repeated before, rarely eliminated ground loop caused hum anyway, it is hard to do without knowing how everything's wired internally. That kind of solution is hit and miss, even if we ignore safety practice.
Cutting the XLR connector's pin 1 to the shield/casing would be the lesser of the two evil, if the OP wants to try, can easily solder it back after.
Or bite the bullet and get an active unbalanced to balanced converter TLSGuy linked before, for $89, it is worth a try, and it should work.
We have to take a step back here and use applied logic, in this difficult case. Our difficulty basically arises from living in different jurisdictions. That is the root cause of our difficulty and why we are struggling with this case.
This is what we know.
The Denon by itself does not hum.
The Classe by itself does not hum.
In Norway, voltage is 230 volts with a neutral and ground. The wall sockets are polarized, but the plugs to devices are not, and are reversible. The sockets are grounded, but devices may be or not.
The Denon does not have a grounding lead in the AC plug, but the Classe does.
Now if the the Denon and Classe have their chassis connected in any way, there is hum. This occurs with no other devices connected and driven from the mains AC.
The core of the puzzle is that it is acting like a ground loop, but where is the loop?
I will tell you where the loop is. It is at the bonding of neutrals and ground at his electrical panel.
With the AC plugs being reversible there is a huge excess of neutral gouging. I have encountered similar situations in years gone by when neutrals were allowed to be daisy chained all over the place, and wall sockets were not polarized. I think most of these situations have been updated and remedied in the US.
So, his ONLY solution is to cut that ground to pin 1 to the xlr body.
Now that has NOT increased the hazard in this case, because the Denon will be connected as intended and protected with so called double insulation. The Classe will be grounded as intended. However, he won't have his hum. The hazard of using the equipment has not changed as everything will be connected AS designed.
I have to comment that electrical codes in continental Europe are just awful. When you leave the port of Dover and arrive in Calais, you move from a country with the highest electrical code standards, probably in the world, and arrive where there is basically electrical mayhem.
I could tell you a few horror stories on my visits to Europe concerning electrical issues, the worst of which befell us in Cortina D'ampere, Italy.
