If you are just trying to power up an amp and/or other devices when the preamp comes up, you can try something like this:
SmartStrip
It uses a "master" plug/socket to sense current draw which then causes it to turn on the other outlets. I built something similar to this to power up my amp since it doesn't have any type of trigger. I used a relay and assorted parts from Radio Shack - the above looks like a more elegant solution for roughly the same cost in parts.
re: the sub
If it's an ML sub, you may want to run pre-outs to the sub and let it filter the bass for its own drivers, then use the return "outs" from the sub to feed the amp which drives the main speakers. This way, your main amp only drives stuff that the main speakers need to handle, and this configuration works for two-channel also.
In particular, for a typical receiver, the sub channel (x.1) is usually not driven unless you are in Dolby or DTS multichannel mode. In two-channel mode, it just sends the full spectrum to the front mains. As above, the full pre-out would be filtered by the circuits in the sub, returning only the non-sub/bass to the amp that drives the mains speakers.