My main concern is knowing how to turn the active sub into a passive sub.
Once it has been physically converted to a passive sub, the internal DSP is not part of the equation since the internal Amp has been disconnected when you turn this active sub into a passive sub.
So no more internal amp and no more internal DSP, which is part of the internal amp.
But I have no idea how you turn an active sub into a passive sub. For one, I think active subs have internal Crossovers. Passive subs don't have internal crossovers.
Just don't power up the plate amp and it's
voila a passive sub once you access the leads to the driver. Just have to sneak in some connection tor an external amp, not that hard and can be done a few different ways. You're not going to connect an external amp to the plate amp, so the plate amp's dsp or low pass filter (let alone a crossover for your speakers) isn't going to be utilized. You can add dsp with an external unit like a minidsp or get a power amp with appropriate dsp on board.
I use Crown XLS1500s on my diy subs (but they don't have dsp for the sub particularly) but QSC and Yamaha are good choices (maybe even Behringer), altho a pro amp's fan noise can be an issue with some models (the XLS are very quiet). The spare channels on your Monolith could well work on their own without risk if you don't get silly with the volume control and ULF content....