To protect the empeg, I'd suggest a SPDT switch with the common going to the amplifier. One side of the switch goes to the empeg remote and one goes to the ignition +12V (I'd recommend adding a 1A or so fuse - depends where you tap it from - if from the empeg harness after the fuse that's probably good enough).

That way the empeg will never see the ignition +12V on its amp line. If you want the amp off, switch to empeg (if the empeg's missing it will turn off, if the empeg's there it will do whatever the empeg wants).