You should get a negative 15volt or so reading across that zener. With the power off, check and see if it is shorted using an ohmmeter or diode tester. Pull it and check again to confirm if it is zero ohms, since C220 could also be shorted. The pass transistor (Q212) would be hot because it would be forced to drop the entire supply voltage. It is possible there is an abnormal load on the -15v rail, but the zener should still have some voltage across it. My bet is the zener has gone short. Replace it with a 1 watt rated 15volt device.
If you decided to bypass the entire standby circuit, you could break into the relay control by cutting the trace, or lifting the collector leg of Q217. Connect a switch between ground and R228 (that previously was connected to the afformentioned Q217 collector). When the switch is closed, that will activate the power relay driver circuit and turn on the amp. The standby lamp and push switch won't function anymore.