adjusting the gain on my LOC

The problem is that you are attributing the problem to the LOC when it could be, and probably is, from something else.  You could eliminate the LOC as the culprit by switching to another model.  Or, just for the sake of erring towards the conservative side, turn all the LOC gains down to halfway or below.  If you go this route you can reasonably be assured that the LOC is not letting too high of a voltage into the amp.  Then go through the process of setting amp gains (again).

But that aside, you are hearing an electrical noise caused by the car's electrical system and is related to grounding issues.  It sounds like a ground loop and could be caused by a number of things.  If you have positively determined that it occurs only with the LOC connected, be sure you have connected it correctly.  You shouldn't be using the ground wire if it has one, which should be explained in the instructions.  I can't believe that the buzzing and static noise is being caused by the LOC's adjustments.  Using a meter at this point to try to equalize the gains would be fruitless.

Start a process of isolating components to find the problem.  Adjust the LOC gains to midway for now and find the deck output level and set the amp gains according to the proven and reliable method outlined here in this forum in many past threads.  Read through the ground stickies at the top of this page, as there are troubleshooting procedures there that will get you on your way through this.

Okay, at this point then you can go ahead and read the output of the head unit voltage through the LOC by connecting the DMM to the amp speaker output terminals.  Turn the amp gains all the way to minimum.  Then read AC voltage and adjust LOC gains until they are all the same.  The voltage is rated per channel, so you should look up your amp's maximum voltage input specs and make sure the voltage is at or below that level.  So if the amp is rated at, say, 4 volts max input  you should read 4 volts or lower on each of the four sets of output terminals.  Rating is usually done at 1kHz, so that would be a good tone to use...(and make sure the tone is a true 0 db reference).  Where there are subs to be connected, however, use a sub Hz tone.  When that's all done, set amp gains.

BTW, the OEM deck will likely start clipping at the halfway point, not 3/4 of the way, because you're using the high level speaker signal (dirtier than line level).  Keep that in mind when you are finding the deck's highest clean output and get it set to that maximum level before you do the metering of the LOC gains.

Oh, and another thing:  make absolutely sure that the polarity of the wiring is correct...at the speakers and at the LOC input and RCA output into the amp.  A mistep here would result in symptoms you described where one speaker is louder than the others.  And always be aware of where the deck fade is set;  if you have two of the channels connected to rear speaker wires, they will have to be balanced to match the front outputs.