When there is hum, a fairly frequent cause is current in the ground lines. This is most often capacitive coupling inside a power transformer, from some high AC voltage such as the primary, to the power supply ground. Since the Stereomour (like all Bottlehead line-powered products) uses the safety ground from the wall socket, any such current within the stereomour is carefully routed to cause minimal hum, but an external source component may also generate such a current. By switching the grounds, that source of hum is disconnected when you are listening to another source. Sometimes you can use that to identify a source of hum.
But if you don't have a source-related hum problem, then switching the grounds does nothing for you. It won't hurt anything, and it is normal in most gear.
