Put a wire from the 12V connection on the 4 pin molex through a relay hooked up to the ignition to the remote turn on on the amp
This will turn the amp on after the Opus has powered up, and when you turn off the ignition, it will turn the amp off. The other option (of hooking the 12V from the Opus direct to the amp) is that it will still "thud" when you shut down the PC, which the method above will avoid
(See attached image)
Garry