Great news - I figured this out and installed a 12" subwoofer running off the M3 amp. Turns out there are actually 4 wires going into the 8" sub, not 3. Those 4 wires are because it's a dual coil subwoofer running 2 x 80w at 4ohms. This matched the MTX 12" sub with box I had in the garage.
The...