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You can confirm with Technic but my understanding of the add-a-sub harness is that it is a LOC (Line Out Converter) so it goes BEFORE your factory amp which in this case still allows your factory amp to be in use for your other speakers. Any processing by the factory amp will only affect the speakers connected to the factory amp. The add-a-sub signal should be a clean/flat signal from the factory head unit.
You can always run test tones through your system to see if you're getting the low frequencies. Just make sure they are lower frequency tones than your low pass point
So with a full signal into your sub amp you'll need to use a low pass crossover(filter) on the sub amp to dial out frequencies above what you believe sounds the best out of your sub(without a REW tune you'd just be tuning by ear and so whatever your preference is). Low pass can be as low as 50Hz (that amp can't go any lower than 50Hz) and some people low pass as high 100Hz. I wouldn't recommend low passing any higher than 100Hz-120Hz because subs make awful-sounding midrange speakers and the higher you make your sub play the more you localize the sound from it, meaning it will pull the sound stage back from in front of you because the sub is playing frequencies your other speakers are playing as well.
A nice thing about that JL amp is that it offers you a choice of low pass slopes, 12/24 dB/octave. 12db is more shallow, 24db is steeper. One is not necessarily better than the other for low passing a sub, just try which one sounds the best. BTW, there is no subsonic crossover on this amp so you won't be able to high pass out any real low frequencies. With a sealed enclosure you won't need a subsonic filter. Only vented enclosers need a subsonic filter to protect the sub from destroying itself below the enclosure tuning frequency.
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