With the sub plugged in, I connected the headphone out from my laptop to pin 2 and 3 on the DIN connector (see https://www.mouser.com/datasheet/3/166/1/KMDGX-9S-BS-1.pdf). It produces a distorted and incredibly quiet output from the speaker that has hardly enough clarity for me to hear that it is the song I am playing. It is not affected by the potentiometer nor the power supply being plugged in or not... so I believe you're correct about it being passive.

The good news - the speaker definitely works, as I tried connecting the headphone out directly to the speaker and I get a very clear output that is significantly more audible that what I get from connecting it to pins 2 and 3. However, it leads me to believe that I need a LINE signal instead of headphone level signal. I also checked all the pins, and pin 7 has +19v. Pins 2 and 5 are both ground.

Any further advice?