youll need a test tone, like 45 hz. turn off the bass boost, leave it off. and turn the gain off.
unplug the sub from the amp,
if you have an oscope it will be more precise, but a multimeter can give a decent way to set the gain
a way to set the gain correct is to run the radio at 3/4 volume with a tone playing and turn the gain up slowly until you get to the correct AC voltage coming out of the amp. to figure out the voltage you need to know the equation for power P= V^2 / R . solve this for V and you get V = sqrt(P * R)
so your amp puts out 1500 rms at 1 ohm. so P * R is 1500 * 1 + = 1500. take the square root and you have an output voltage of 38.7 volts
so with the multimeter set on ac volts, put the neg in the neg speaker terminal and the pos in the pos terminal. then just turn the gain up untill it reaches 38.7 volts.
Im not sure if that amp is over rated or not, but this should get you in a good ball park range.
And once you do this leave it, if its not as loud as you like it, youll have to upgrade to a more powerfull amp. You dont need anymore blown subs