If you are REALLY serious about getting more light out of your
headlights, look into making a wiring harness. Many times the wiring to the
headlights is under spec'd which causes a voltage drop.
Taken from Daniel Stern Lighting:
In many cases, the thin factory wires are inadequate even for the stock headlamp equipment. Headlamp bulb light output is severely compromised with decreased voltage. The drop in light output is not linear, it is exponential with the power 3.4. For example, let's consider a 9006 low beam bulb rated 1000 lumens at 12.8 Volts and plug in different voltages:
10.5V : 510 lumens
11.0V : 597 lumens
11.5V : 695 lumens
12.0V : 803 lumens
12.5V : 923 lumens
12.8V : 1000 lumens ←Rated output voltage
13.0V : 1054 lumens
13.5V : 1198 lumens
14.0V : 1356 lumens ←Rated life voltage
14.5V : 1528 lumens
As you can see, if you battery is at 14V (a common voltage) and your wires are dropping the voltage to 12V you just lost 40% of your light just in those 2V. Similarly, by increasing the voltage you exponentially increase the light output.
A harness is EASY to make if you are interested and I can provide you with the needed schematics to make it.
Just tell me your bulb size(s) and I'll whip one up. I did this on my car before I did a retrofit and I must say it made a noticeable difference.