Grounding Question

@chayro Lifting the AC ground is a thing that people do to eliminate ground loop buzz. Its an indication that the grounding scheme of one or more components in the system isn't well thought out.

You can test for this sort of thing using a Digital Voltmeter set to the Ohms scale. The test applies to equipment having a metal chassis.

You connect one probe to the RCA ground connection and the other to the chassis. If the reading is the same as when you connect the two probes to each other then the equipment you are testing has a grounding problem.

Older equipment like the Dynaco ST70 didn't have a 3-prong AC cord. So on account of being vintage it gets a Murphy. But for newer equipment with a 3-prong AC connection there really isn't any excuse.

How does new equipment designed without a ground in the power cord play into this?

@atmasphere  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. 

Thank You @erik_squires!
Finally, someone with a little snippet of Electronics Knowledge.
You just brought up one of my Pet Peeves. Just about every one of the Commerciale Power dist. box manufacturers will try to convince you that they can carry sufficient current for your equipment and yet they almost all exclusively use just a tiny copper trace on a circuit board to deliver all of the power.
On another point I agree that you should NEVER depend on your inner connects to carry the ground. Though most of the time the transients that they do carry can safely handle, it is definitely insufficient to carry a serious load. BUT Most equipment that doesn't haver a dedicated ground wire/pin do utilize the Neutral side of the power cord as a pseudo ground. There is lots of debate over how much is enough or too much grounding. You can go too far and actually create ground loops which will actually DESTROY your sound.

Modern electric devices without a ground prong on the plug are double insulated. They're constructed in such a way that they're safe without one. Audio aside, this is my preference for any power tool.