I guess #3 fits for me.
I have now have a line out converter connected to the output of the factory Blaupunkt and down to my 2 channel amp, whose line outs are connected to my mono sub amp.
The 2 channel amp's speaker outs have 14 gauge wire going back to behind the HU and from there they connect to the factory speaker wires.
I really considered switching to 14 gauge all the way from the amp to the front speakers, but when I saw that the rubber duct was actually a connector harness with NO open space to feed wire through, I changed my mind. I didn't want to mess with that, and I didn't want visible wires going from the car to the door.
I'm a middle aged guy, with a middle school mind.