Right, thanks nap! And if I would like to transmit signals over the power lines as with powerline networking it's up to me to deal with filtering the noise, because I am asking the electrical system to do more than it is designed for.
Ground loops pose problems because the bonding conductor(s) have too much resistance producing voltage drops that are large compared with the magnitude (often less than one volt) of the audio signal. A #14 copper wire daisy chained among and securely attached to the chassis of each piece of electronic equipment almost always cures ground loop problems.
When the transformer secondary is isolated, it may still be connected to the transformer primary starting with the secondary terminal connected to the the electronic circuitry ground connected to the chassis connected to the ground wire of the power cord connected to the ground wires of the electrical system connected to the main panel neutral/ground bus bar connected to the neutral wires of the electrical system connected to the neutral wire of the power cord (finally connected to the neutral terminal of the transformer primary).
Transmitting signals over power lines depends on the signals being far enough removed from 60 Hz with the latter being filtered out by the receiving circuits. Since there may be harmonics of 60 Hz, most likely 120 Hz or 180 Hz or 240 Hz, in the power line, having the desired signals somewhat higher than that is desirable so those frequencies, too, can be filtered out without deleting portions of the desired signals.
Usually the signals are confined to the power lines connected to the same side of the 120/240 volt feed coming from just that one utility pole transformer because the latter usually won't pass the signals on to other part of the power grid. For practical purpose within a home, the signals can only propagate through about half the branch circuits namely those on one side of the 120/240 volt service. A special filter circuit can be put in the panel to get the signals over to the other side of the 120/240 volt service.
Tornado victims: Do not rush to rebuild. Take your time and look for and get a good contractor. Or consider selling the property and moving to a home that is ready to live in.