Thanks guys. I did some more testing, this time with a MP3 player jacked into one of the RCA inputs on the amp to be sure if the amp was on or off. The trigger will turn the amp on the first time but after that it won't shut off again. Even though the trigger was causing a clicking noise in the amp, the music from the player was still being amplified through a speaker, and the front light remained blue.

I don't know if this is an issue with the 7500 or the PR-SC5509 processor not sending the correct signals. How do the 12v triggers work, is it one pulse each for on or off, or a continuous voltage for on / no voltage for off?

(Edit): I should add that if I started with the amp switched off (orange light) then plugged in the 12v cable and turned on the processor - nothing would happen. If (after turning the pro back off) I then pressed the amp power switch the amp would stay off with a orange light. Turning the pro on at that point would turn on the amp (blue light). From that point the amp won't turn off, even by pressing the power switch. I have to disconnect the cable then press it to get it to go back to orange.