Effects of magnetic fields

That's an interesting question. Any flow of electrons has an associated magnetic field. Any magnetic field will have an effect on the flow of electrons in it's vicinity. That's how transformers work and how magnets can affect a picture tube.

So the short answer is yes, all of these fields and flowing electrons interact. Is it audible? It depends on the strength of the signal being affected compared to the orientation and the strength of the magnetic fields trying to affect it.

Put a phono stage dealing with very small signals next to a big power amplifier with large transformers and it is very easy to hear the humming that results because the large fields from the amp transformers induce currents in the phono stage, but the phono stage will have a much smaller effect on the amp.

It is directly proportional to the strength of the field, the distance betwen the devices, and how they are oriented.

Changing the position of your speakers will change how they sound mostly because of how they are interacting with the room and their relationship to your listening position. Im most situations these factors will far outweigh any effects due to magnetic fields. All of your equipment is also affected by the vibrations in the room and the strength of these vibrations is different at different points in the room.

If you move equipment other than the speakers and the hum level goes up or down then that is probably due to magnetic fields, but if some other aspect changes with no difference in hum levels then that is probably due to differences in pressure (vibration) levels.

You are correct, there are many variables and it is difficult to isolate them.