Impedance load

OK, that amp is rated at 360 watts RMS X 1 into 4-ohms.  So, disconnect your speakers from the amp, set your amplifier gain to minimum, turn up yout head unit to it's maximum output volume before clipping (which you should have already determioned before you begin, and if not try about 75%), play your 50 Hz tone, connect your multimeter set to AC volts (RMS if it has such a setting) to the amplifier output terminals and slowly turn up the gain control until you see no greater than 37.94 volts.  This will be equivalent to 360 watts output into 4-ohms.  Be sure to turn the head unit off before you re-connect your speaker leads.

To check the voltage output on yout RCAs, connect one meter lead to the outer ring of one RCA (red or white it doesn't matter) and the other lead to the inner socket or pin.  Read it on AC volts scale.

The Kicker guy was right in saying that.  He was referring to setting the amp gain to match the deck output.  But here is where it gets a little confusing:  look at the gain control.  Most amps it is a screw that turns clockwise.  All the way to the left, or counterclockwise is thought of as off, or at its lowest point.  You are starting there to begin matching gain to the deck output.

Now here is what is tricky.  The gain at far left is actually for high voltage input.  And the more you turn it clockwise, the gain is matching lower voltage.  So far left is maybe 10 volts, and far right is 1 volt.  If you are matching 2.7 volts, the amp gain screw will be turned clockwise about 3/4 of its travel.  As a contrast, my HU is factory, and I get about a 9 volt signal from it to the amp, so I find that my amp gain is barely moved to under the 1/4 area to match the input.

This BCAE page  explains it really well.

I read the voltage with the speakers hooked up.  Because I thought  the amp won't make voltage without a resistance load.  Read on AC volts at the 200 level.  With a test sine wave,  I got an unwavering reading.

I also read the input voltage of the deck  at the amp speaker terminals, with the amp gain off.  I don't know if this was correct, because the reading was lower than I thought it should be, but I got a volt reading.  Like I said earlier, it was interesting, but then I set it by ear.  Actually, I set the gain more clockwise.

You asked earlier about hearing distortion, or clipping.  When your testing the limit of the HU, listen to the front speakers.  There will be a strong and steady increase of volume as you turn up the dial, but at a certain point you will notice the sound gets fuzzy and the volume doesn't increase at the same rate.  That is the distortion point.  Keep the sub off while you do this.

Then, when your listening at the sub for distortion, unplug the front speakers.  Adjusting the amp gain, you will again get to that point where the sound is dulled, not crisp, somewhat fuzzy.  That's that point again.