Since it's normal for a car's electrical system to vary over a range of 10.5 to 14.5 volts, this is not exactly a trivial matter. A variation of this degree in the average house voltage of 115 would cause a fluctuation from about 100 volts to 140 volts. Your refrigerator, color TV, and computer would definitely show you the meaning of one form of instability. Another form of stability is related to load impedance. Many installers are somewhat familiar with this and are constantly performing speaker impedance calculations to determine the proper way of connecting speakers to amplifiers.
Every amp has design limits that are based on voltage and current limits of the output stage.
Higher impedance loads require higher voltages and lower currents for a given amount of power. Likewise, lower impedance loads require higher current and lower voltage for the same amount of power. For example, to drive an 8-ohm speaker at 200 watts requires that the amp produce 40 volts and 5 amps. In each case, the power produced is the same, but how well the amp can produce the 200 watts is dependent on how well the load is matched to the output stage of the amp. If a speaker requires 40 volts and it's matched to an amp that can only produce 20 volts, then it will fall short of the required 200 watts no matter how much current the amp can produce.