Ok I was just checking this setup out, I have been out of the game for a while but do you think a 50 pound sub is a good idea to toss into a thin box like that? I guess I would be concerned that it might just pull right out of the enclosure with a few good speed bumps lol.
What wires are you tapping for your line out converter? When I had just the stock sub wires tapped, the bass output was very low and low only. That's why I went back and tapped one of the rear door subs in conjunction with one set of stock sub wires. This gave way better response. Try also not disconnecting your stock sub and running it together with your aftermarket sub. My head unit sound levels are bass: 0 mid: -1 treble +4
Also make sure that your amp crossover is set all the way up to 250Hz and that your bass boost is set all the way down.. as that usually increases only the lower bass around 40Hz.
About to rewire and install a new sub and custom box.
Which speaker to tap with the stock sub, Front door or rear door speaker???