I'd guess an active sub can tailor its frequency response in the electronics. It's quite possible that the power amplifier reaches full power in the 20-30Hz range at levels far lower than would result in full power at higher frequencies. In other words that spec. may only be valid at moderate sound levels.

Also consider whether you really need 20Hz. The fundamental of the lowest note of a piano may be 27Hz, but most of the sound is in the harmonics and overtones up to 500Hz and higher. If you reproduced that piano note though a system that cut off below 50Hz, you'd hardly notice any difference. Double basses, bass guitars and big pipe organs might produce significant energy below 50Hz, but for most real music the majority of sound amplified or recorded in that region is wind noise, air conditioning and floor rumble and best filtered out.

Most of the value of a sub is to provide clean power in the 50-150Hz region where there is significant musical sound energy, taking a huge load off your main speakers so they can deliver much higher power without distortion.