I thought I'd share my idea with you all incase anyone else wants to do the same. I have wiried up a SSR (solid state relay) into a metal box to be used as a 12v trigger for my Onkyo 818 receiver, however my receiver only has triggers for zone 2 and 3. I don't really like the idea of having to do extra button presses of having to select zone 2 (or 3) with the remote, then hitting the power button to turn on the sub. Same thing for turning it off. This partially defeats the point of having a remote trigger because I may forget to turn the sub on or off.
I looked at my receiver for anything that had switched power and the USB ports (front and back) stood out to me. They both turn on and off with the receiver so I am going to use them to remotely trigger my SSR. USB provides 5v and my SSR only needs 3v, so everything should be fine.
The only issue I see, however it won't affect me, is that turning on the receiver's Zone 1, 2 or 3 turns on the the USB ports. This means that if you use zone 2 or 3 for another room, the sub would turn on. I don't use those zones so that isn't an issue for me.
I plan on swinging by radio shack to pick up an 1/8" male connector and taking a mini or micro USB cable and hacking the 2 together soon, I'll update with how it works, however it should work fine.