@zipost
There is no universally correct polarity option, it’s all system dependent. The same directions apply, flip the switch until the sine wave is louder at the listening position.
I’ve also never dealt with subs that need external amplification, so I can’t help you there. However, DSP is usually a huge part in both subwoofers that come with their own plate amp and even DIY subs, I’ve never heard of a sub where you can just add any amp and it’ll work optimally.