crosen,

For line level inputs, since their input impedance is typically very high, current is very, very low, therefore power is low and input voltage is the limit. Mild overload will simply sound bad, major overload, such as a static discharge, could result in physical destruction of circuit elements.

On another level, lets say that the gain of our gadget is 10x. If the power supply voltage is 10V, the output cannot go beyond 10V. Therefore 1V would be the maximum input before clipping. (Note that it is not generally possible for output to exactly reach the power supply "rail".)

From a system design standpoint, the line level input will have a maximum input voltage spec that should be respected. The output from this line level device should match whatever the follow-on device needs. If the follow-on device is a loudspeaker, then significant current is involved and power and cooling should be discussed.