Well ... I decided to buy a Kicker 8" Hideaway sub and install it, for now.
I found this web page, where a guy installed a Bazooka bass tube in his Z, which motivated me to give it a shot:
Bazooka Sub install in 370z with BOSE. - Ontario Z-Car Owners Association
Unfortunately, when I took mine apart in my '16 Nismo, I realized they've changed things around! As per the attached photo, you can see that my Bose sub doesn't have the separate amplifier inside it anymore. The harness that plugs into the outside of it only has 4 wires, which get passed straight through to the 2 subwoofer speakers in the enclosure.
So is there still a separate subwoofer amp in my car, hidden someplace else in the vehicle? Or did Bose just use one 6 channel amplifier now to run ALL of the speakers, including the sub?