Knowing the age of this passive sub and the configuration of connections on the back, I was thinking the 12" driver might have a dual voice coil. This video confirms that.
https://www.youtube.com/watch?v=FRdfX4RE-ZA
A mono connection will only power half of the driver's voice coil, and connecting both inputs of the sub to the amp in mono mode will create an impedance problem for the amp. It will be best to connect the amp in stereo mode to both inputs on the sub.
https://www.youtube.com/watch?v=FRdfX4RE-ZA
A mono connection will only power half of the driver's voice coil, and connecting both inputs of the sub to the amp in mono mode will create an impedance problem for the amp. It will be best to connect the amp in stereo mode to both inputs on the sub.