Sub design confuses me. How can a 12" sub be on par with two 15" subs?
The size of the woofer and its other characteristics are separate issues. For example, some woofer cones are capable of moving a greater distance than others, and this will be relevant to the performance of the speaker. Do NOT buy a subwoofer based upon the size of the woofer; buy it based upon actual performance.
Another thing that many people get confused about is power with a subwoofer. A more powerful amplifier is NOT a sign that the woofer is louder or deeper or better in any way. Assuming that the power ratings are derived in the same method, a speaker with a 100 watt amplifier could play louder than another one with a 1,000 watt amplifier. Or it might not; it will depend upon the efficiency of the woofer in the box. So, do NOT buy a subwoofer based upon the power of the amplifier; buy it based upon actual performance.
Actual performance, of course, involves frequency response, volume output (how loud it plays), and distortion. Typically, manufacturers are not forthcoming with all of the important information, so one tends to look for professional reviews in which actual measurements were taken.