I don't have a schematic or the amp in front of me so I have to speculate.
Sorry, the 1455 is a dual 555.
180mV is pretty high for a headphone amp.
With the protection circuit engaged, you should see nothing at the headphone connector.
The relay could be implemented as either open circuit or crowbar.
Open circuit, as the name implies, keeps the headphone jack disconnected until the
protection says it is safe to connect it.
Crowbar shorts out the output until the...