After seeing, and participating in, the discussion on 3-wire cords, and seeing other discussions on grounding in general, I think that relating a few experiences may be useful. First, a few things:
a) I have always, well, at least since I was 22, lived in houses or apartments with radiators on cast iron risers from a central boiler that was also directly connected to the (copper) water service. Meaning that I have had a good ground available in every room of the house/apartment.
b) In general, I have linked all my audio components, even those with 3-wire cords from the factory, together and thence to a good ground. For those components not equipped with an actual ground lug, I have used a spade lug on an exterior screw in some unobtrusive location.
c) And, just for common sense, kept the path as short as possible.
d) Using fairly robust fine-stranded copper wire, usually 12-gauge.
I have never had any hum problems due to this method. And on at least one occasion, when a very-vintage power-transformer chose to fail on a very vintage, very cheap solid-state amp, the fuse blew on the amp rather than giving me or anyone else a shock.
When I was living and working overseas, our villa had central AC, all ductwork, no suitable, similar local grounds available but the ground in the receptacle (NEMA style). Being the boss, I had our electrician drive a ground rod outside (even though we were in a desert country we were very near the Gulf, so the water table (brackish) was about two meters down). From there, I came up in insulated #8 stranded wire to my work bench, and to the living room for the grounds, one for my hobby, the other for the stereo.
Point being, after all this, I would not rely on the third leg of a receptacle being always a “good ground”, especially as compared to another receptacle or other local ground such as a water pipe. Nor, these days, would I rely on a water pipe much as with the advent of shark-fittings, PEX, dielectric unions and other devices and materials, continuity is often interrupted.
And, especially in older installations, BX (armor-flex/MC) cable relied on the exterior armor for the ground, and older NM (Romex) permitted the ground wire to be one gauge smaller than the primary wires. Such grounds are adequate for appliances, tools, and similar, but not so much for audio.
As things stand right now, I use the heating system standing radiators on black-iron or copper risers, verified all the way to the main municipal water service as grounds for the five operating stereos and my workbench, as described above. No hum issues.
I very strongly urge individuals who own/operate/maintain audio equipment, especially older/vintage/legacy equipment however well restored to install a separate, high-quality ground rather than rely on the mains ground, which can be problematic or worse. Keep in mind that the tools that verify correctly polarized receptacles and grounds are pretty broad-brush devices, such that resistances of a few dozen ohms will not show as a failure – but could be damaging to equipment, or its users. I know that this might be difficult for renters or apartment dwellers. But in those cases, be absolutely sure to make sure, and be sure, again, that _all_ your equipment is connected to the same ground and not several. And if all your equipment shares the same preferably dedicated circuit, that is even better.