Three things you can calculate instead of guessing: what voltage to set the gain to with a multimeter, how thick the power cable needs to be, and what fuse belongs in front of it.
The amplifier
use the rated RMS figures, not peak power
W
Ω
V
Amplifier class
Setting the gain with a multimeter
U = √(P × Z)
Play a sine test tone from the band you actually listen to — 50 or 60 Hz works for a sub — and make sure the file is at 0 dB, not quieter.
Turn off everything that boosts or distorts: bass boost to zero, EQ flat, loudness off on the head unit.
Turn the head unit up to where it still does not distort — on most units that is about 75–80% of maximum. From now on, that is "full volume".
Turn the gain all the way down, then measure at the sub terminals with the multimeter set to AC volts (V~ / ACV).
Bring the gain up until the meter reads the target voltage above. That is it. Gain is not a volume control — it matches the amp to your head unit's output.
Keep the measurement short. At full power a sustained sine tone can cook a voice coil in seconds — music never looks like that. Measure in bursts of a few seconds and let it cool.
Most handheld multimeters are accurate around mains frequency, 50–60 Hz, which is another reason that test tone suits this job. If your amp will not produce a stable signal without a load, leave the sub connected.
Power cable and fuse
sized from voltage drop
m
m
V
Cable material
How much louder will it get?
quick table, before you spend
Change
Gain
What it means
2× the power
+3 dB
1000 W instead of 500 W. Barely an audible difference.
4× the power
+6 dB
Now it is obvious — but it takes four times the amplifier.
10× the power
+10 dB
This is what reads as "twice as loud".
2 subs, same power
+3 dB
More cone area from the same watts.
2 subs, double the power
+6 dB
This is why it pays to think in pairs.
Sub with 3 dB more sensitivity
+3 dB
Free — you just have to shop more carefully.
These are free-field, theoretical figures. In a car, cabin gain, enclosure tuning and standing waves at the seat will redraw all of it — but the order of magnitude holds. The lesson is usually that the enclosure and the wiring buy you more than a bigger amplifier.