line converters are a joke. as far as subs go, as most stock hus have xovers built in to minimize the sub 100hz frequencies to the rear speakers.

there were 2 sets of wires on my back 6x9s that one was for the bass that worked with the gain slider and one was for higer notes. If you tap into the lower one it wont be that bad. yah I know that it will not be amazing but neither is anything else unless you upgrade everything. If you wanna try it go for it. Just saying for a stock hu the system wasnt really bad. If it has bass blockers in the line you can just cut those out too. The way I have installed them in cars they didnt sound bad like you guys are saying. I have a friend i wired up with 2 MTX 9500s and the second largest amp MTX makes and he has a stock headunit for now because he wants to get a good hu and doesnt want to buy a cheap one now since he spent all his money. It was really good, not the best but it gets the job done if you need a fix for now, or dont need perfect quality.