Grounding Question

 can you elaborate on your last statement bout two pronged older equipment? I have an amp when you turn it on you get a ground loop hum hd it goes away when the preamp fully turns on. 

@dinov That doesn't sound like a ground loop at all.

Normally the procedure for turning on any audio system is to power up the preamp (or in pro audio systems, the mixer) first. After it has stabilized then you power up the amplifier. In this way you avoid buzzes, thumps and the like as the preamp turns on.

When the preamp is off, it can act like an antenna for buzz and hum. When its turned on, its a much lower impedance so the antenna thing goes away.

erik, I'd bet that your sub has more to it than merely a wooden case. The electrical portion is likely isolated on plastic. Wood isn't a suitable insulator.

If equipment is designed with 3 prong plug (Class I Equipment) case is grounded protecting user from dangerous voltages.  Grounding the case thru interconnect to another case is not only unsafe because of the limited current interconnect can carry, but also for the simple fact that interconnect can be disconnected making device unsafe.  Two prong plug devices (Class II Equipment) offer user protection by double insulation.  This insulation has to withstand 3000VAC.  Wood alone is likely not suitable insulation since it can absorb moisture.

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.