subwoofer amp with crossover/phase control

@ricred1  
 
The slope is to match the roll-off of your speakers. Ported and passive radiator speakers roll-off at 24dB/oct, sealed speakers at 12dB/oct.  
 
After finding what frequency you want to cross over at (80Hz for instance), next you find the phase; play a sine wave at the frequency you cross over at, you can find clips on YouTube or on websites (like this one), and adjust your phase/polarity until it registers the loudest in the seating area (so it requires two people or leaving a SPL meter in your seating area), you want the loudest as that’s when the phase is alligned, any misallignment causes a reduction in output, and fully opposite in phase completely negates the sound.

@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.