One of the main functions of the power transformer of an audio component is to change the nominal 120V line voltage to the appropriate AC voltage for the circuit application, further refined into the appropriate DC voltage for the design requirement by the rectifier circuits.
The output voltage of the power transformer is a ratio of the input voltage. So, for example, a sudden doubling the input AC line voltage (from 120V to 240V for an instant) will also double the output voltage from the transformer.
A well designed circuit will have a significant overvoltage margin designed in to keep the unit alive when a line surge occurs, but there are limits to the overkill protection that is reasonable to provide. Thus it is possible for a significant line voltage surge to produce an output voltage that can overvoltage and kill rectifier parts, and even generate an excessive DC level downstream before the rectifier dies, killing circuit parts too.
Good design will provide internal fusing and other transient protection to provide as much protection as possible. But even good design has limits.
Lightning strikes of course produce much higher voltage spikes, which hopefully will simply take out suppression capacitors on the incoming line side to ground before doing any other damage.
Note that vacuum tube components, that are running at higher internal voltages in any event, may have to tolerate really huge overvoltages from line surges. Fortunately, the tubes themselves are very rugged in this respect, but big power supply capacitors may not be as lucky.
I suppose that a really expensive line regulator might be a great idea to keep audio equipment alive under all possible circumstances. But not if it is a lot more expensive than the equipment it is supposed to protect.
Frank Van Alstine