Bob,
We could program you up a second hand AMX controller to plug into the RS232 port on the 568, detect when the 568 has a source selected using its RS232 output (so we know its been turned on) and then flick one of its internal relays to provide you a 12 volt trigger for the 558. Turning this relay back off when the 568 goes into standby. You have another box and PSU(The Axcent 3 and its PSU) which you have to put somewhere, but you don't need to get at it for it to work. It's a bit overkill but it's an option.. cost shipped would be about �250.
Rick