Unfortunatly. having worked as a commercial sound technician for many years, the FIRST thing many people do when there system hums is yank the grounding pin. Audio signals (TRS & XLR connectors) are usually tied in with the electrical ground of the sound system. This creates a loop, which can cause a hum. The proper way to get rid of a hum is to transformer isolate the audio line, or ground lift the audio line itself. Leave the electrical connection alone. Sorry. This has always been a rant of mine.
As for the outlet in the bank/shop, isolated ground outlets are nice when wired right. Whoever used a red wire as hot should be shot. People like me get sent to the hospital because of stunts like that.
-Ben
