I am assuming you are using an ATX power supply? (You said old, but just how old a PSU?)
You can look here for info on how to use a psu with no systemboard/motherboard.

As for the amplifier power on, are you using a docking sled to "slide" in the empeg? If you do, obviously, like your car, you need to connect the blue wire, the amp remote line. If you are not using the docking sled, you could create a toggle switch, for when the empeg is slid in, to turn on the +12v dc from the psu to the amp, along the remote line.
Anyone want to double check this? The remote line on an amp is +12vdc, right?

And now for something completly different.....

Smoker_Man
#080000449 MkII - 36Gb Blue