Have you looked at the preamp's manual? It appears the 12v trigger has an adjustable delay. Did you wait long enough for it to activate?
12 volt trigger not working
i had been using a Niles AC-3 voltage activated AC outlet with my previous Preamp. This allowed me to have my amp turn on when I turned on the Preamp by running a connection from the AC-3 switched outlet to the 12 v trigger on the Preamp. When I tried to duplicate this with my new psaudio BHK Preamp, the amp does not turn on. I tested with other equipment and the Niles works fine. Is there something I need to do to activate the 12v trigger on the BHK? The Preamp apparently does need 100 mA to activate the Niles and I do not see this spec anywhere in the PSAudio documention. I will contact psaudio on Monday since they are closed now but hoping for some thoughts.