Converting a cheap Class D amp into a subwoofer plate amp – is it simple?

Noob question and not even sure in which forum thread to post – so forgive me if it has been dealt with before, as I could not find answers on the forum yet. I have a cheap TDA7498 2x100W amp board and would like to try it for subwoofer or helper woofer duty in a FAST setup. So, the plan is simple – to drive stereo woofers (which I don’t have yet) up to say 200Hz. Above that it will be full range drivers on a different amp.

Many of the cheap TDA3116 2.1 amp boards have three on board pots – L/R volume, sub volume and one to adjust the cut off frequency, which I assume is an adjustable low pass filter? If this is the case – can I add some circuitry to my Class D amp to also do this and is it that simple? So, can one build a simple adjustable low pass filter say up to 200Hz and hook it up to the amp to effectively create a subwoofer plate amp? Does this filter work on the incoming (line level) signal, or on the outgoing amplified signal?

I know that miniDSP is a better solution, or - why not just buy a plate amplifier, or - why not make a passive line level crossover, are all valid arguments – but right now cash flow is a problem and I have this Class D amp already and only need an adjustable low pass filter with it to do what I want.