The reason car stereo speakers are 4 Ohm impedance is to increase power. Maximum power delivered to speakers is V*V/R(Z really). You either increase output voltage or lower the speaker impedance for highest power. All that is beside the point. There is only so much volume any human an tolerate. OP speakers would have to be unusually inefficient to require more power to achieve desired volume. That is why I suggest measuring actual DC current to get the picture.
I have Bose home audio system in my motorhome powered by an inverter and its contribution to total current at 12V nominal is less than 2A at any volume. Measured continuously by Blue-Sky IPN-PRO with shunt.
