[TheSoundSteward]

One big clue to your problem is the date your house was built --1924.

It is likely that the power may on "tube and post". Long before Romex was invented the AC wiring was installed with bare wires about a foot apart on glass or ceramic insulators. Because of the huge spacing between the two current carrying conductors, there would be almost no cancellation at all of the magnetic field that surrounds each wire. A real nightmare of hum-inducing radiated magnetic field. If the neighbor's place is wired the same way, you will have the same problem from the top. If no current flows in the wires, there is no field. It makes it hard to record and play with the circuit breakers (fuses) off.
Using a battery powered amp (for a test) like the Radio shack audio amp $13 that runs on a 9 V battery would allow you to cut all the power to at least your place to confirm that the problem is magnetic radiation from the power wiring. It may be that the main tube and post feed goes up and down inside one of the walls, maybe an outside wall and then is tapped into at every floor to feed the apartment on that floor. You may be getting some magnetic radiation from the wall. If you can use a pickup to trace the path of the wires, it will give you some clue.


To Fix it.

AC wiring should have Hot and Neutral (and Ground) held in close proximity. Ideally the hot and neutral should be twisted and the ground wire not. Romex does a decent job of approximating this condition. Most of what I suggest is easy in new construction, but may be impossible in rework without making a big mess.
But I will give you my best shot at theory.

If you use Romex, you are done. Route it somewhere besides under/over where you play your guitar.

If you use individual wires, they must be put into conduit. Use STEEL, not aluminum, flex to contain the twisted wires. Nobody twists the wires, but the extra 1/2 hour you spend twisting will significantly reduce the radiated field, even in the Steel flex. Electricians hate Steel because they have to use a hacksaw to cut it instead of a pair of cutters and it is heavy. It is no more expensive than aluminum and it offers about 6 dB of magnetic shielding. Not much, but better than none which is what aluminum gives.

There is a product called BX armored cable which is a steel flex with the wires already twisted inside. You may have seen it used to wire up a garbage disposal under the sink, if yours doesn't have a cord and plug. This stuff already has steel and twisting going for it.

The worst situation is where the AC loops around the room as others have mentioned. In your case, perhaps after abandoning the AC wires that are under your floor you can re-route them near the walls which already have sockets in them.

All this is a royal pain, but it does significantly reduce the problem at the source.

Best wishes that you can find a relatively easy solution.