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Actually 2 12s will give similar output as a single 18 on the same power, if you gave twice the power you would get louder than a single 18 and you can go higher than twice the power, likely 5-6 K is possible. A single 15 you can do but due to the significant difference in cone area you would need a lot of power to reach a 18, it would be very expensive not for the amp, amp is cheap but for the electrical, the 15 would likely really need twice the power to rival a 18, about 3dB is what a 18 has on a 15. If you still want a 15 check out steve's videos on the new deaf bonce 15s, looks like those subs are highly efficient due to the gigantic motors, no t/s params for them yet but for sure will be one of the loudest 15s ever out there.

In your situation I would go for 2 12s, say the sundown M12s, total cone area is near a 18 and 4.5 cubes net and 70-80 square inches of port area will be enough for them. You can then upgrade the power later if you want and go louder than the single 18 would be able to. Wanting to have a 15 to sound as loud as a 18 on the same power is rather difficult, you usually need like twice the power on the 15 to reach the output of the 18 and then it can be done but it is more expensive, you need a better electrical, amp and sub.