Insufficient information exists. For example, normal is for a surge be incoming to many adjacent appliances. But only cause damage in one. The surge is incoming to everything but only does damage in one or a few parts inside one appliance. Yes, it may pass through maybe 15 parts. But only the weakest part fails.
Relays open or closed are a conductive path to any surge. A common path through a relay is from its coil to its wiper contacts. Open that relay and find no indication. Normal is for destructive currents to pass through most items without any indication. Even open relays.
A surge can also connect to concrete via the wire and not leave any indication on wire insulation. Observation alone is one reason why many never understand how a surge does damage.
Demonstrated by the IBM PC and so many other engineering examples is the near zero damage created by fields. The resulting energy is big time too small. A nearby strike only ten feet from a radio antenna can create thousands of voltage on that antenna's lead. And an NE-2 neon glow lamp (less than a milliamp) attached to that lead then converts thousand of volts to tens of volts. Near zero. Because fields create a high voltage with near zero current. Nowhere near sufficient energy to create damage as described.
Lightning can also create other conductive paths causing much higher energy from AC electric to then create a follow through current. That follow through current is one reason for more serious damage. And an example of why damage is traceable to a human mistake that let a surge inside the building.
To have that damage means a current had to be incoming on one path and outgoing on another. Routine is to have a surge incoming to every appliance in the house. But only one appliance is damaged - ie the amp. Because that one appliance made a best connection to earth. Because that one appliance acted like a surge protector for everything else - the preamp, tuner, tape deck, refrigerator, answering machine, computer, bathroom GFCI, mobile phone on a charger, and even smoke detectors.
Damage always means a current that was incoming on one path while simultaneously outgoing on some other path to earth. Damage that exists only because the surge was not earthed BEFORE entering the building. And that is a common human mistake. To not earth every incoming wire either by hardwire or via a protector.
Too many possible and unreported paths can exist. But we know this from well understood energy numbers and over 100 years of experience. Fields do not and cannot cause damage you have described. And you do not know all the many and possible paths and electrical conductors that exist from the appliance to earth. A surge can even pass through wire insulation without leaving any indication.
You have wires entering the building without first connecting to single point earth ground. Not safety ground. For many electrical reasons, safety ground and earth ground are different. Even when interconnected. Even wire length (ie 'more than 10 feet') makes the two grounds electrically different. TV cable connected to safety ground bus bar in a breaker box) means it is not properly earthed for electronics protection.
Adjacent surge protectors never indicate 'spent'. That light only reports when a protector was so grossly undersized as to disconnect internal parts as fast as possible. To otherwise avert a house fire. Nothing on those protectors report 'spent' - also called degradation. The light only reports when a protector was grossly undersized and was a threat to human life.
View how a protector is wired. Surges at thousands of volts can pass through those protectors. And only create tens of volt differences across the protector parts. An example of relevant electrical concepts that cause many to ignore significant details. You have assumed protector lights report things that those lights can never report. A protector could be completely degraded. But its light will still report good. Light can only report one type of failure.
Without actual inspection and other details, nobody can say exactly how a surge found earth. But this much is obvious. You did not have properly earth protection on every incoming wire. Somehow a surge current was all but invited inside where it found a destructive path to earth. Damage, as described, can only occur from something with higher energy. Induced fields can never create that damage. Otherwise every RF amplifier in every radio (something far more sensitive) is destroyed.
