Setting gains, A8002T

Voltage = square root (Watts times Ohms).  400 X 4 = 1600, square root of this = 40

So if your load is 4 ohms, your amplifier has to produce 40 volts to be at 400 watts. 

Set amp gain at minimum.  Wear hearing protection.  Start the vehicle.  Play an appropriate test tone from your HU (is this a subwoofer?  Try something between 50-70Hz.)  Turn up volume on HU until you can detect the tone has changed or gone "shrill" or "flat."  This is the distortion point on your HU and is the MAX volume you cannot exceed without causing possible speaker damage.  Back off the volume until the tone is clear again.  Memorize this volume setting.  Turn off the HU and the vehicle.  Disconnect your subwoofer from the amp.  Connect your DMM set to AC Volts with the positive meter input connected to the + speaker output and negative to -.  Start the car and turn the HU back on.  Play the same tone and turn the volume up to your earlier memorized setting.  Slowly turn up the gain on your amplifier until your meter indicates 40 volts AC.  This is the max gain setting to match the HU output.  Mark it with a sharpie.  Leave it there and never touch it again.  Disconenct your meter.  Turn off the HU and the vehicle, reconnect your subwoofer to the amp.  Fire it up and see how it sounds on your music.  If the bass is overwhelming the mains, you can back the gain setting DOWN to balance the system, but NEVER turn it up past the point you set with your meter.

Yes, that's what it means.  If you adjust the amplifier gain higher than the HU voltage match point your amp begins clipping.  For the normal, non-competition user, there is no way to "tweak" the electronics beyond this point.  It is possible to gain SPL by simply adjusting the position of your subs, or yes by changing enclosures (sometimes.)  But you cannot squeeze blood from a stone, and you cannot squeeze more from an amplifier than it is designed to produce.  If yur gear does not give you what you want, often the only answer is different gear.

Now I will say that in SPL competition there it is normal to run amplifiers in clipping modes, but this is the kind of situation where the goal is one massive burst and if the speaker frys the speaker frys.  Normal installs do not operate like this and most people do not want to purchase new speakers every time they turn their system on.

Set it to AC VOLTS.