Sounds like the ground shield on one of your interconnect cables or jacks is failing mechanically. Could be there's a strain relief problem when the cabling is attached to the unit. Are the interconnect or speaker cables particularly heavy or otherwise straining the terminal posts or receptable?
My guess is that it's something mechanical. After all, if your father's an electrician, it's unlikely that hot and neutral have been switched at the outlet. Still worth double-checking.
Say, have a friend connect up everything, under your guidance. Watch them do it. Does one of the connectors "give" a bit, or require a lot of force to connect the cable?
The shock you're receiving could be leakage current, low voltage but enough current (milliamperes is enough) to give you quite a surprise. Regular electrician type equipment may not be adequate for this task.
Could also be that the chassis of one of the components in your system is "floating" way above ground. You should be able to measure this voltage, relative to ground.
Another idea: the case of output transistors is often "live". Could one of them be shorted against a heat sink?
When you send the gear back to Klaus, be sure to include the power and interconnect cables.
