Grounding Question

Ground loop hum would not go away as you describe. You don't have ground loop hum. You have another problem. And your question about the interconnect is complicated.

All components even with just two plugs are grounded. It is not a Murphy, it is just the way electrical panels are wired. Black is power, white is neutral, or utility ground. Millions of homes and older appliances wired this way, perfectly safe. The third plug is earth ground, redundant since neutral is already ground.

Vast majority of components every speaker output and RCA in and out, they are all grounded to the same chassis ground. Look inside, all the RCA are wired together. Ditto speaker terminals. Since this is the same on both components then yes connecting them with the interconnects connects the grounds and if one is earth grounded (third prong) then both are.

Where it gets complicated is the whole point of that redundant extra earth ground (third plug) is extra safety, which in order to work it needs to be able to carry a lot of current. Which is why the ground wire is always the same gauge as current. Not so the interconnect. So yes it is grounded, but no it isn't. If you think this correct answer is hard to understand, just wait till you see how thoroughly messed up it gets from here. Electricity questions are just the worst! Makes me want to bang my head into a Max Headroom sign.

@erik_squires

I don’t know where You got your Electricians license or your Electronics Eng Degree, but I took my training at USC! Now, I do admit that I did take my schooling a long time ago but US Electrical Code has changed very little and Electronics has not done anything other than to Improve on a theme. If that confuses you, Well, you can’t be any more confused than I am over your statement. You mixed metaphors so badly you were almost impossible to follow. Talking about 4 prong plugs. I hate to tell you I don’t know of ANY city in the US that uses a 4 prong plug for 110v circuits. SO your 220v and 440v discussion is irrelevant, at least for those of us in the USA. Then talking about a wooden case, HOW does that even enter into the argument. The standard for electrical plugs is the NEMA 1-15 plug.

Now I just did a continuity test on a 2 prong IEC C7 socket to chassis on my not too old SACD player and it OHMed out at near ‘0’! Similar on my computer chassis to motherboard. I will agree that on the computer, they have through the last 20 years played with isolating the motherboard from the chassis, but that is no longer the case. Also I am not saying that it the Neutral side of the IEC C7 plug is meant to function as a full on Ground, for overload purposes. There exists Two standards for the socket for the IEC C7 and one is reversible and the other is keyed for polarity. Even on my BRAND NEW NAD, the IEC 60320 plug doesn’t have an active ground pin so the 3 prong cord is not even active.
None of you statement seems to be germane to the conversation.

Devices fitted with two-prong plugs often have a signal ground available (e.g., a dedicated ground lug on the rear panel, typically used with turntables, in many cases), which should be utilized and should eliminate hum. If not fitted with such a connection, verify chassis (using an ohmmeter) is electrically tied to low side of input/output connections (i.e., very near 0 Ohms) and connect components together using an external wire or braided cable. If there is more than one (common) path to the power line ground for your audio equipment, you will likely have hum and/or perhaps a potential shock hazard. If your system includes devices that employ both power and signal grounding schemes, you'll need to trace out which is which to eliminate hum and/or shock hazards, if they exist.

The low side of RCA (unbalanced) connections are typically tied to ground (power or signal, depending on construction) internally, with balanced connections having a dedicated ground. These grounds are signal connections and shouldn't need to carry excessive current as mentioned previously.

Also keep in mind that for 120 VAC in the US, the neutral (white) is tied to ground at the electrical panel. Many years ago, my folks purchased a cheap, Western Auto stereo (not double insulated) with a two prong plug. When my brother and I used it, the chassis would 'float' at 120 V, shocking us if we were grounded. If the plug was reversed, operation was normal.

Good Luck!

My friend had used a cheater plug for a decade on his RAM-9 amp.  I told him to remove it and also replaced a very old cheap power outlet (I gave him a Black Synergistic Research plug).  The difference in his system is phenomenal.  Never use a cheater plug on tube amps or pre-amps.  Probably never use them because they usually are made of inferior/garbage quality contacts, case, etc.  

If it came with a two prong power cord get an AFCI/GFCI combo receptacle. Sure are a lot of clowns on this site!