What have you been smoking?
Any sub of any size can play any frequency at any loudness like the next sub whether 12/15/18/22/etc...
If you have a 6.5 in the smallest box possibly built with 10k+ power sent to it, yes it will indeed play louder and as low as needed, but it specifically would have to be a sub and not mid range speakers.
Now an 18 with the same power will play just as loud with a 2 decibel difference on a meter, but the difference will be it will last longer aka not burn as quick due to coil sizes.
All the op has to do is make his box smaller and tune the same with the more power and he will get way louder without new subs.
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