Well,
A line driver basically only sends a higher voltage signal to the amp. This CAN help eliminate noise, but won't change Frequency Response.
Keep in mind that if your amp can only take a max input = to that of the max output on the HU, than a line driver will serve no purpose. If you have an amp that can accept a 10v input, than you may want to consider one. If you don't have noise in your system now, there isn't much value in a line driver.