First, you need to understand how the 3 wire grounded circuit works. The hot side (120 volt potential, and yes the narrow blade of the polarized plug) delivers the current. It is usually fused the minute it enters a piece of equipment and then goes to a power switch, then to a transformer. All stereo equipment works on DC voltages, so that is why there is a transformer. The neutral is connected to the ground in your electrical circuit breaker box and is used to complete the 120 volt circuit. The circuit breaker box has 2 120 volt circuit in it. When you connect the 2 120 volt circuits in series, you get 240 volt, which is used to run your electric clothes dryer or kitchen stove, which requires high amount of current to operate. The third wire, the ground also goes to the circuit breaker box and then goes outside to a very long copper ground rod that is pounded into the ground outside of your house. The size of your electrical service is determined by 1) the gauge of wire to your house from the electrical company, and 2) the size of your mains breaker.
2 prong wiring to lots of equipment, simply is all that is required. The is little to no risk of electrical shock, under normal, expected operating conditions. As an example, a vacuum cleaner does not have a ground wire. It's usually used on carpet, which is an insulator, and the vacuum is usually made out of plastic so there literally is no way to complete a circuit, and get electrocuted.
Toasters are 2 prong, and usually made of metal, but they are used in the kitchen where GFCIs are required. GFCIs monitor current from hot to neutral, and if they are not equal, the GFCI trips, cutting all power to the outlet.
Now, most hifi equipment is made of metal, but you as the user are expected to operate it within the home, not on wet concrete floors, or use said equipment with frayed power cords. They are safe as long as you abide by these simple rules. When you decide to take your receiver outside in the rain, that trouble can certainly come about.
Three wire cords for amps certainly are now popular, as are IEC sockets. But actually, all of these amps could have gotten a UL listing with just a 2 prong plug. The manufacturers that promote the 3 prong are using it as a marketing tool.
Also, the three prong plug can lead to ground loops for hifi equipment. If each component has it's own ground and if there is even the smallest milliamp leak of current to ground it will create hum.
Wayner