Yikes! The third prong (ground) on the power cord should never be a problem. If it is, this indicates that the manufacturer has not done their homework and figured out how to ground their equipment properly.
Using a cheater to look for ground loops is OK, but it should not be used on an on-going basis. There are certain types of failures that could result in fire or shock hazard. The equipment should be fixed! So if you have an amp or preamp that hums when the ground prong is plugged into the wall (and this will usually only manifest when connected to other gear) it needs to be returned to the factory and reworked.
If they tell you that is not possible (for example: "thank the UL Laboratory" or some nonsense, like that 'hum' switch on the other thread) sell it and buy something else that is properly engineered! Seriously, there is no excuse for this.
Using a cheater to look for ground loops is OK, but it should not be used on an on-going basis. There are certain types of failures that could result in fire or shock hazard. The equipment should be fixed! So if you have an amp or preamp that hums when the ground prong is plugged into the wall (and this will usually only manifest when connected to other gear) it needs to be returned to the factory and reworked.
If they tell you that is not possible (for example: "thank the UL Laboratory" or some nonsense, like that 'hum' switch on the other thread) sell it and buy something else that is properly engineered! Seriously, there is no excuse for this.