Those sub amps in DefTech speakers are junk. Saving the speakers is complex. We just went through this with Mythos STS super towers
The problem is that I can't find any measurements of your speakers to find the crossover frequency and slopes.
You can not just do what you propose, as the crossover point to the subs is above sub range. So you have to get a sub that can be crossed at the right frequency to the mids, and set the receiver to LFE + Main. Then we have to know the crossover point to know how to set the receiver crossover. In addition you will need two subs, each one placed right by a speaker, because of the high crossover to those small mids.
In any event this is honestly a poor solution. So if you don't want to do an exchange on the amps with DefTech, which I think is $300 each amp. You get warranty if you measure the DC resistance of the woofer, if you don't you get no warranty.
With those, I think the better option is to junk the things, and look for better speakers.