I would check the electrical polarity or ground of the systems you are using. You can use a volt/ohm meter for check the polarity between various amplifiers you are using and especially between the amplification and the PA system. I’d always touch the strings of my guitar with the microphone to see if any sparks would fly. This should definitely be checked when using American equipment with transformers in Europe as the voltage is much higher there.

Often the ground is lifted from the strings and the control cavity of an instrument. This can increase unwanted electrical interference to the amplifier that would normally be reduced by touching the grounded strings. I’ve seen fuse circuits installed between the strings and the grounded controls in an instrument. If you happen to touch a ground with reverse polarity such as a PA system, it will help reduce a dangerous shock.