I haven't used the newer generation Outlaw amps, so I'm not the best source of information on the 7125 and its current siblings. My 750 behaves differently than I think the current amps do (I have to have the front panel power button pushed in for the trigger to work). My understanding of the 7125, however, is that the front panel power button is a momentary contact switch such that this step isn't needed. I'd suggest checking the Yamaha's setup to see if you have to enable the 12V trigger there in order to make sure the signal is being sent to the 7125. If the trigger signal was absent, then the 7125 would behave exactly as you describe.