first thing you have to understand is there is ZERO industry standard around power rating for an amp. 700watts, 7,000 watts. They can write what ever they want.
in reality, it is much closer to 13 watts a channel.
using simple"ohms law"...where Power (P) = Voltage (V) * Current (I)
12 volts * 20 amps (max fuze dedicated to stock audio system) = 240 watts.
240 / 9 channels = 26 watts @ peak output. Consider half that when running nominally.
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Bottom line.....you can never have too much power. The reason factory radios sound like shit is because they have too little power and the amp goes into clipping which destroys speakers. the ****** stock speakers will play fine with more power, but engineers really dont understand how audio works and look to save money on drivers.
Too be clear....the amp is the weakest link, not the speakers.