Agreed about the DC supply not being either practical or necessarily an improvement, particularly after going through a DC to A/C converter. My only thought was to eliminating interference generated by other equipment plugged into the same electrical circuit. But if as you say, boss, what really counts is what happens within a metre of the equipment being fed, then perhaps it's not significant in comparison to other factors, such as those you raise above.
When you mentioned passing traffic, I immediately thought of mechanical vibration - passed through the ground and through the air - but in context to exposed wiring in the equipment, you obviously mean radio waves emanating from the ignition system? I can perhaps understand radio waves effecting electronic audio equipment, but mechanical vibration I can't, except to the extent that it may affect the stylus tracking through the record groove. Do you mean perhaps that the vibration of the equipment, and cables particularly, causes nano-scale voltage peaks caused by the ever so slight movement of wires through a magnetic field? Whether mechanical vibration or radio waves, I don't see how AVM could possibly prevent either effect. To work against mechanical vibration, the AVM would have to sit as a buffer between the part it was protecting and the source of vibration; therefore, if ground borne vibration, solely on the feet of the equipment or furniture; or if airborne, you'd think everything would move including the AVM. If it's radio interference that the AVM shields against, then how? my understanding is that only a metal shield will work.
Perhaps thermal vibration is the chief culprit, but doesn't hi-fi equipment sound best at "operating temperature"? Otherwise, you could use a cooling system to keep everything chilled for superb sound.
As for the battery powered headphones sounding better after midnight, maybe there's less ambient noise to interfere with your hearing, or maybe there's less airborne electromagnetic radiation because more equipment is turned off in the neighbourhood.