Achieving perfect input voltage

According to the JL Audio website, both of your amplifiers are switchable from 200mV - 2V to 800mV - 8V. It's not a variable range, its either/or.

And with you using the LC6, I really don't think you'll be able to drop the input voltage to 2V, so I would say set your amplifier input switch to 8V and adjust the LC6 accordingly.

In reference to adjusting, there is really no "Perfect Voltage" to adjust to per say. The best way is how the manual is telling you to, The "maximized" light is a clipping indicator. Therefore, you don't want it to stay on, you want to minimize the flickering, setting the signal to its maximum without clipping your amplifiers.

Well to get it perfect you would want to get a test speaker.  Make a voltage divider.    Get a test CD that has constant 1000hz  track.    Hook up the test speaker to your head units speaker output. play the track and when you hear the signal clip(it will be noticable) then you will know where your head unit starts to clip signal.   That is the max you should want to ever turn your head unit up to.  

  Now install the voltage divider to your test speaker and then hook it up to 1 of the channels on your 4 channel.       Play the track over again with the head unit turned up to the number at which it started to clip on the first test.     Then adjust the gains on the amp until you hear the same noise you heard from the 1st test.      That is where your amp starts to clip.   

  You can repeat the last process on all of the channels to be sure they are all set evenly.     Same goes with your sub amp.    Just be sure to use the test speaker with a voltage divider installed because otherwise when your setting the gains on the amp that freq will kill your hearing fast not to mention possibly cause damage to your speakers.       

Dont get me wrong the audio control units light is probably pretty darn accurate but that would be the sure way to set your gains properly.  

actually ....

I would assume that if you just hooked up a test speaker to one of the channels on your amp and used the test disk.    Have the amp gains set as low as possible and then turn your headunit up until you hear it clip, that would be the best way to test for clipping on the headunits low-level output.   The run the test again with the voltage divider installed and turn the headunit up to the previously discovered limit and the adjust your gains on your amp until you hear the cliped signal again.