Re: Speaker 101
↑ What he said.
As for your math... component speaker systems come with passive crossovers that filter highs from the mid bass drivers and lows from the tweets & the amp doesn't see the impedance of the speakers, it sees the impedance of the crossover. So, even though there are two speakers connected to each channel in that scenario, it is still a 4Ω load on the amp... presuming the component set is 4Ω.
And if you want to find out if the rears are connected to the head unit or the amp, unplug the RCAs (with the system OFF) running the front channels and listen to it. If only the rear speakers and sub are playing, then the rears are connected to the HU. If only the sub is playing, then the rears are connected with the fronts.... which is doubtful.
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