Amp 12V trigger
Does that amp have an input for a 12V trigger?
If so, you can use it to turn on the amp. Look for a 12V trigger ouput on the receiver. If it doesn't have one, you can make one by connecting a low power (250-500ma) 12V wall transformer to the receivers switched outlet on the receiver.
Some amps also have the capability to detect when a signal is present (or not) and go in and out of standbye mode. This feature may make the trigger unnecessary.