Confused about system
If what you're telling me is correct you're trying to drive a stereo sub -woofer with a mono amplifier. If so, that could be a bad situation for the amp, if the voice coils are not wired together correctly. Before we go too far, I have some questions I need answered.
1. Is the MA unit an M1889i?
2. Do you have the 4 terminals of sub-woofer tied together. For example the 2 positive terminals are tied together & the 2 negative terminals are tied together?
3.How do you supply 12VDC to the amp?
While I'm waiting to hear back from you, I'll try to draw a schematic (wiring diagram) of the way it should be wired for your particular setup that I'll send to you.