immathewj- Your old vintage components are made that way because that’s the way pretty much everything was made back then. That’s the way the US electrical grid was designed to be run. Over the years though people get less and less educated, more and more heavily regulated, until today we are afraid of our own shadow. Redundant earth ground is now code in most places, and some are starting to add an even more redundant third ground to plumbing. When we see people attaching copper ground wires to plastic pipes we will know we are at Spinal Tap 11.
Eliminating, cutting, or "floating" the redundant third earth ground is one way of solving ground loop hum problems. In your case, with older gear with just two prong AC you already are "floating" the ground.
Power conditioners, almost all of them, primarily use a large transformer. These transformers are designed to be very efficient at 6oHz. AC is 60Hz. Ever notice makers of tube amps and other things like to brag about their custom wound transformers with great top end frequency response? Because it is hard to make a transformer do high frequency. Radio frequency is in mHz- mega. Not kilo, mega. So AC coming into the primary is chock full of RFI, but the AC coming out the secondary has a lot less. This is pretty much a power conditioner in a nutshell.
There is no "isolated" with running lines. Everything plugged into the outlet is connected to everything everywhere else plugged into the grid. All you have to do is follow the wires back to the panel, remove the cover and have a look inside to see that this is the case. Everyone else’s home is wired just the same. All those wires connect to neutral, and they all go back to the incoming utility. Everything is connected to everything else.
The only true isolation is a battery disconnected from AC and housed along with the component in a shielded chassis. I have this with my Rens Heijnis modified Strain Gauge phono stage. It works pretty good.
See my simple test above. Turning off two of your circuits will disconnect those two hot leads, reducing by that much the length of wire collecting and funneling RFI into your system. To that degree it will help. You might even be able to hear it. I have done a similar test, turning off some breakers and then others, and there is a difference. The more the better. Just remember to turn them back on. People get so cranky when the hot water runs out.