I have my cabinet set up with one momentary switch (button) that starts and shuts down the computer. There is also a automotive relay (ford) that switches the monitor and amplifier. I simply ran 12v from the computers power supply to the relay and soldered two of the wires from the monitor and amplifier to the relay, and the other two wires to the 120v input.
When the computer starts, 12v is sent to the relay bridging the contacts, which turns the monitor and amp on. once the computer shuts down, the 12v is cut to the relay, and the monitor and amp shut off as well.
I'm using a 30 amp relay, which is well above the requirements for the monitor and amp. If I had more room, I would've done a powerstrip setup, but since I've got a cocktail, there isn't enough space for one.
-Harry