Before you start buying new things, I would diagnose the problem properly first. I re-read your original post, you seem to be having problems with output from your sub? Could be several things. One, you might be expecting too much out of it. Two, there may be setting on your HU that might be misadjusted. Doublecheck all signal processing on the HU. Also, you said the stock speakers distort. Are you saying that you can't turn the volume up any higher than 18 or 21 because of this? If so, you need to high pass your mids. Better yet, get an amp that has a built-in crossover for them. 1kw to a 15" sub with stock speakers and HU power is a bit much. High passing the mids will limit bass frequencies from overdriving the small mids, therefore, allowing you to turn up the volume higher without distortion. Whatever is the case, a line driver isn't your solution.