i have seen, and experienced amps smoking for not apparent, controllable reason. sometimes transistors just fail, and a resistor out of spec can cause this, as well. not the most likely but does happen. anyways, what happens with a voltage drop really depends on how the amp is designed, or what type. keeping it simple, we'll look at pwm, power supplies, which are real common. what they are doing is switching 12v back, and forth through one side of the transformer, usualy, each direction is under 1/4 of the time on. it monitors the other side of the transformer to keep the voltage up to a certain level( rail voltage). when you have less voltage, then there is less power coming out of the 12v supply, then it has to work harder, and keep the pulses on longer, to pass more power and keep the rai supply up. it will max out at 50% cycle. the transformer relies on a charged rail to keep resistance down on the primary side. as that is depleted, it allows more current through. past this point, there is voltage drop on both sides. it is real similar to a low battery turning the starter in your car. at higher voltage it passes an acceptable amount of current, and stays cool enough while working. a half dead battery barely turns the starter less, and drops resistance, passing more current. you can often see starter damage coincide with a weak battery, or recent weak battery. similar principal with transistors.
