Most like drivers are a straight up GIMMICK!
1st bad thing about a line driver is if you aint already clipping your preouts out of your headunit, you will sure as hell be clipping the preouts coming out of the line driver which boosts the signal, which results in your amp clipping than leads to your speakers clipping and potentially blown woofers and possibly driver or outputs of your amp..
Next what does a line driver really do besides boost the preout voltage of a headunit. Often times almost everyone uses the line driver wrong by installing it in the rear of the vehicle by the amp, not close to the headunit like they should be if they would take a few minutes to read the instructions. Hell Ive been guilty of this before, but you live and learn.
Anyways.... Continuing on..
Take a look at almost any decent amplifier, even a lot of your cheaper amps and knock of amplifiers.. They all have an Adjustable gain. That gain is to be set with your preout voltage, its not a volume knob.
The amplifiers gain on most amps are adjustable from 0.2volts up to 6volts, sometimes even 8 or 9 volts depending on an amp.
What does that mean?
That means that as long as your headunits RCA pre-outs are anywhere between 0.2volts and 6volts, that you can properly tune your amplifier for full unclipped undistorted power if you have the correct tools ( Oscilloscope, Digital Multi-meter, SMD DD-1, etc)....
Almost every radio, even your cheaper Duals, Vr3, Sony that you buy at Walmart have 1volt or 2volt pre-outs, which means the line driver will serve you no purpose or gain in performance. Now your nicer radios or higher end radios will have 2volt, 4volt, 5volt, 8volt preouts, again making a line driver useless!
The only time you can really see a gain from performance from a line driver is if the line driver has a decent built in crossover network (such as some of the AudioControl units but all cost $200+ at minimum).
Or if the line driver accepts High level inputs, so you can convert a highvoltage signal from a speaker wire into a lowlevel signal, than boost and reshape that signal due to the loss of power when its converted, and restore freq response that are often missing caused by a factory radio. Many processors will do this, as well as a few other units like the piece by Cache, or even the new SMD-LD1 iirc which will be a distortion free line driver that accepts high level inputs.
Another reason a line driver may come in handy is if you are doing a crazy install and say you have 10 amplifiers that you are using for you subwoofer stage alone and have to use a bunch of RCA Y-Splitters to divide the signal up into every amplifier resulting in drastically reducing the radios preout voltage (but than again almost all your newer amps have RCA inputs and outputs or can be daisy chained to eliminate the need for y-splitters.
Other than the 3 reasons above a line driver will not gain you much of anything besides having a distorted clipped output after installing it.
If you use a line driver, that means you have to turn your amp gain down, or else you will be clipping.
No matter if you add a line driver or not, the line driver if properly setup will not make you louder.
The line driver will not make your amp operate more efficient.
The line driver will not make your amp have more power.
The line driver will not make your amp run at a cooler temperature.
Again if the line driver is properly used and tuned, you will notice/hear/see no difference to your system at all on 95+% of the systems out there!!!!
/essay