Don't doubt the American Pro as a beginner amp. Mine pushed my hifonics 12s pretty hard before I burned it up from running at bridged at 1 ohm. Lol. That first system bring back so many memories. I actually thought I was pounding.You may have better luck trying to power sub off the headunit. Those American Pro amps literally don't look like they have more power than a headunit.
And be sure that you don't have a loose battery terminal or have it wired into the incorrect factory wires.