OK. You have determined with a voltmeter that the horn NAC is not supplying signal power. Test one of the spare circuits that has a 10K resistor connected with your voltmeter. With the panel in standby mode there should be supervision polarity present. When the panel is put into alarm the polarity on the meter should switch to alarm mode polarity. If it does move your horn circuit wires to that circuit. Then test the horns.
Is the “strobe plate” you are referring to is the 4050-80?