Not exactly a nightmare, just "international design differences", so to say. Local standard is 3-pins, afaik "we" even invented that 3-pin Schuko plug. But, as not every device uses it, problems are foreseeable.
Luckily it's only a handful of audio devices that react to this. As I already sold the amp, the subwoofer is my only device left with a "modded" plug. As it comes with a wooden case, the risk of a lethal stroke of electricity is fairly small.
I've tried several things already to get rid of the problem before I took the third pin out of the equation. Different wall plugs were the first thing. There's nothing coming in from an antenna or any other device, just the fact that the sub is connected to the amp and both use different types of plugs.
The same thing happened with the headphone amp, it's connected to my PC and both are using different types of plugs.
Wall outlets didn't help, I tried different power strips, even master/slave ones, without any success. Different cables, orientation of cables (yes, some manufacturers have this in their FAQs to cure the problem), nothing.