Here's how to automatically turn on & off an external audio amplifier with your PC...
For audio in my new office, I have a 1000-watt Rotel RMB-1075 5-channel THX-certified amplifier that's hooked up to a pair of extremely capable Hales Revelation Three speakers. I connected the output on my computer audio card to the pre-amp, and voila! listening to music on my PC from my 120 gigabytes of music is a delightful experience.
Using a Kill-a-Watt meter, I found that the amp consumes 80 watts of power whenever it's on. So while in general, I just turn it on and off when I come in and exit, I remembered that it has an "external trigger" on the back. I wondered if I could somehow have the PC turn the amp on and off automatically.
After a few Google search revisions, the search for "trigger pc audio amplifier" found this excellent article:
turning on power amp via pc -12v
Fan extension cable (for connecting to the fan header/supply on the mobo):
It's simple enough: take a 12V source off your PC (in this case from a connector on the motherboard fan), wire this up to a female 1/8" plug (which is attached to a hole on a chassis plate), then just run a standard male-to-male 1/8" cable out to the amplifier's "trigger in" port.
I will try this and post results when I have them.