I don't think it is a European issue specifically, especially if it wasn't humming when you brought it to your tech (whom I'm assuming is running off the same power grid/sources). So unless for some reason the power being sent to your home is somehow different than what your tech is getting, I don't know what could be causing it in that respect other than something is off with the power source you are using which is causing some issues with the voltage draw or other factor inside the amp. Tubes amps are unfortunately very picky with voltages and power. Have you reached out to Mesa? Did they provide any type of solution or guideline? From my experience they can be really helpful with most issues. I'd recommend contacting them directly - yourself - describing the issue and see what they say.