Defensive line is a slider that works in conjunction with mentality and closing down and the effects are only visible when you do not have the ball and you are defending in your half.
Pushing a defensive line slider up will result in a greater number of goals that are created from runs made behind your DLine by the opposing wingers and forwards. I can see the Dline effects quite clearly.
If you are looking for the line per se, you won't find it. Its not easy to spot, and it takes some time to understand. You guys are about 6 months late into the argument on the defensive line. Older threads include tonnes of tests, scenarios and discussions. Initially I was one of those complaining about how hard to spot it is. Now I'm out.
Defensive lines also work differently for different formations, a 352 and a 442 may not appear to have the same kind of dline...cos they both need different kinds of set-up.