Gain controls on an amplifier are basically just small potentiometers (variable resistors) or volume controls if you will, that allow you to adjust the incoming signal to the amplifier so the amplifier works well with your headunit of choice or to match the level of other amplifiers in your system.

Its not rocket science to set the gains. Gains are like little volume controls, (I don't know why so many installers are taught that gains are NOT volume controls, when in fact that is EXACTLY what they are!)  its super simple to just set them where the level sounds good to you.

With one amplifier its desirable to have a nice swing on your headunits volume control. Let me try to clarify this a little. 

If we hook up a head unit with a 8volt (or more) output to an amplifier, then the volume will get loud very fast when we start to turn it up...In other words if our digital volume control goes from 1-30, then a HIGH VOLT output to an amplifier might make the amplifier reach full power at 5 on the volume scale... That kinda sucks cause it would be nice if you had a little more swing in your 1-30 range! 

And by the same token a headunit with a LOW VOLT output might have to be turned up all the way to 30 and might still not quite drive the amplifier to full power... That sucks too!

A gain control in this case will allow you to adjust the amplifier so it allows the volume of a headunit to control the amplifier so it will get loud at a desirable point in the 1-30 swing... Usually about 3/4 the way up. We don't want it to get loud too fast as we wont have a good control as music levels differ. And we don't want it to have to be turned up all the way to get loud either, because since different music may be recorded at different levels if we set the gains for max output with one music source it might not get loud with a music source recorded at a lesser level.

So, by setting the gains so 3/4 turn of the headunits volume knob gets it LOUD gives you plenty of control and some extra above the 3/4 mark in case you get some music that's recorded at a lesser level... 

To do this its easiest to do it by ear. No need to drag out the TEST TONES and OSCILLOSCOPES!  They will do you absolutely no good. 

One MYTH is how the gain controls will help to prevent amplifier distortion and amplifier clipping...  That's simply not true, UNLESS you set the gains at a level where the headunit cannot possibly drive the amplifier to full power.. And even if you were to find this magic spot for your gain controls then (A) you would have to turn that volume control FULL SWING to get your system loud and (B) since many music sources (or disks) are not all recorded at the same level, its likely that if you have a disk recorded lower then you cant get it loud at all! and if you have a disk recorded louder then you can still surpass your magic spot... So in reality searching for this magic spot is fruitless! Dont waste your time...

In the early 80s when high fidelity car amplifiers were just starting to make the scene I worked with a pretty crazy installer that was kind of legendary around these parts... I wont mention his name but he was pretty highly respected at the time.. Well anyway, this crazy installer had heard that the amplifier gain control was to prevent amplifier clipping.. (still widely heard today)..  Well this crazy installer set up EVERY CAR WE DID to the point where the gain control was so LOW that if you turned the head unit all the way up the amplifier WOULD NOT DISTORT.. And of course if you did turn the headunit all the way up the system would just be getting loud... 

Customers would find that some cassettes would be recorded at a lower level and the music just wouldn't get loud enough... The Crazy installer would FLIP OUT and tell the customer that a REAL AUDIOPHILE doesn't want his music to distort or be that loud!  The customers were NOT HAPPY and came to me to say "Gee Eddie, I don't want to make the other guy mad but can you adjust my system so it sounds good and please dont tell the other guy?  Of course I said yes, and some of those customers from back in the early 80s are still my customers and they are sending sending their children to me for work as well.

SO, you see the only way the gains can be used to eliminate clipping or distortion will also limit your top end volume! And for most of us it is NOT DESIRABLE to do so.

As long as this is not done, it is just as possible to turn your system up to FULL power and beyond to clipping no matter where the gains are set....

Now, on to another reason to adjust a gain control.  A MULTI AMP SYSTEM! 

If there is more than one amplifier it is possible that one set if speakers plays louder than another! This can be because of mounting location in the car, it can be cause the speakers are different sizes or different brands or maybe your two amps are different brands with different sensitivities, either way, with the amplifier gain controls you can set the radio fader in the middle (if you have one) and then make the adjustments to the amplifiers so the levels are the same.  Good thing those gain controls are there...

This multi amp adjustment is pretty easy to do by ear, simply have someone sit in the listening area and tell you as you adjust them when the sounds are equal. Simple! 

All the VOODOO about watching an oscilloscope and looking for a clipped signal is a waste of time and wont get you what you want anyway!  Do it by ear, its simple. Don't let adjusting the gains scare you, they are simply volume controls...