The sub is a passive 8 ohm dual voice coil (2x8ohm 2x100W) i.e the speaker (sub) has 4 connections rather than 2 you see on a single voice coil speaker and doesn’t necessarily dictate L+R channels.
The benift of subs having 2 voice coils is the vast number of ohme combinations possible from using one or more speakers and how they are wired. Using a resistor if needed to show loading on one set of the + – terminals of the amp, which could be done at the sub end, would make it possible to add another sub in a more appropiate area of the boot, whilst retaining the factory fitted sub and enable it to be put back to factory state in no time.
Sorry for the shitty drawing but it gives a few examples.
