Here's my 2 cents. My apologies for being long winded.
I live in a vintage guitar amp world and have been doing 2 to 3 conductor conversions for the last 20 years. These are recommended for the safety of the guitarist and they can aid noise problems with guitars (which are basically a noise antenna). Guitar amps differ from hi-fi systems because they typically have much higher gain. Imagine adding another gain stage or two in front of the phono preamp.
The 2 conductor amps use a "grounding" capacitor between the chassis and the power cord. The ground switch allows the user to select which line, the hot or the neutral, is connected to the chassis through the capacitor. In the event the manufacturer omitted the switch to save a buck, the plug would simply be flipped over in the outlet to select which line is connected through the ground cap to the chassis.
Properly used, the switch connects the chassis to the neutral via the cap. This provides an audio ground through the house neutral back through the panel to a ground rod or water pipe. It is not a "hard" or DC ground, but is through the cap, making it a signal ground. From an audio perspective, the guitar strings won't have noise on them because they are grounded.
Putting the ground switch in the wrong position would have the hot line connected to their chassis (and the guitar strings) and users would get a shock when touching something grounded, like the microphone or another amp or PA. The cap limits the current at 60Hz so these shocks typically are not fatal. When these caps are misconnected in this manner and they short, a bad situation is created. A three conductor cord can eliminate this cap, for improved safety.
Grounding guitar amps with a three conductor power cord is advised if the amp is going to be used (i.e., not a museum piece). This lowers the noise floor a bit compared to using the ground cap and improves safety in the event of a component failure like a power transformer short to the laminates. Amps are quieter with grounded cords and grounding improves the noise from guitars, many of which are noisey themselves.
FYI, the grounding lug is required to be crimped to the green wire, not just soldered. This is typically secured to the chassis under a PT bolt.
Hum issues that are created by the hard grounding the chassis (provided the outlet is wired correctlly) are due to ground loops created by plugging the guitar into other grounded equipment, example is the use of an A/B/Y switch and two grounded amps. The solution is a better A/B/Y switch with an isolation transformer although lots of guitarist compromise the ground to break the loop, jepardizing their safety somewhat.
Because of my experience with guitar amps, I believe the hum issues experienced by some in hi-fi systems converted to 3 conductors is due to ground loops. I would expect a system with only a single active unit, like a receiver with no other sources connected, to have no hum created with a proper 2 to 3 conduction conversion AND a properly wired outlet. Connecting a CD player or other device adds the possibility of ground loop problems. If this happens, the problem is not the properly grounded receiver, it's the other device. But lifting the receiver's ground will break the loop and the problem goes away.
