Gain does not make your amp play louder. Your amp can only put out a certain amount of power. The gain is what allows you to match the head unit's rca preout voltage.
As you turn up the volume on your head unit, the rca's outs send the music signal to the amp. this voltage is increased in the amplifier. this happens until the amp max's its power capacity.
IF you max the amp gain, your amp maxs output at like a quarter of the head unit volume. Not good. It does not get louder and then your amp starts to clip and sounds like ass.
Now your sub level control at the headunit just increases the voltage of the subwoofer RCA. Couple that with max gain and you are throwing the max power of the amp with clipping at your subwoofers. So max heat as well. IF your subs are not designed to handle it, you will smoke them
and possibly the amp if a coil melts and shorts... It's not pretty.
Here is what I tell guys when they are wanting incredibly loud bass.
Pay for a good box design and as big of a sub as you can fit. Multiple subs are great because it spreads the power over multiple motors (coil magnet of the sub).
If it's still not loud enough, all it takes is money to get louder. If money is an issue, be happy with what you have. AND save up for what you want.
I use my son as an example because he is/was the typical kid that wanted tons of bass.
He turned 16
we set up a 1000 watt Sundown amp on 4 eight inch subs. 80 watt per channel X4 channel on everything else. He did a very respectable 139db at the local car audio shows.
He changed cars due to an accident
Now he bought a Buick park avenue
sold his 8's bought four 10s
same sub amp
It was pushing a 142 with the change of sub sizes
He graduated high school
I got him 2 1500 watt sub amps (JBL GTO14001)
told him to run just one of them because his electrical was not going to cut it
2 replacement alternators and 3 battery's later because he waould not listen, he finally came to me for help
the electrical needs of those amps would literally kill his car. voltage drop would hit 9 volts
So he had to spend money on a high output alternator and 2 extra batteries in the trunk.
he swapped to 2 twelve inch subs and metered a 149
He can jam all day. We did install a battery tender in his car so he plugs it in at night and keeps all three batteries fully charged. Which helps in the winter a lot!!
AND now its a new car...
I found some old pics from the install a couple of years ago
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