The only problem with a line converter is the signal they pick up is already distorted by the inductance of the speaker and the output quality of the system. In my experience, anyways. I noticed a HUGE difference when I got a sub preamp output from an aftermarket receiver. But that was a while ago and maybe they are better now.
Why not just get an amp that has a speaker level input and skip the converter? I do like Pioneer gear, so
. Remember to set the sub amp to low pass filtering. I do agree with a closed box, since you don't want a crazy amount of bass and it will sound better for rock anyways.