Why would a 21 have anything to do with lows?
It is a stigma people have with bigger woofers, they assume the bigger the woofer, the louder lower notes get. While typically true it is for a completely different reason. Which is partially caused by mechanical advantage/disadvantage. Just enjoy bass!
Lower Frequency Response does play a little bit of a factor in the lows. A woofer with a FS of 30 hertz will play the tone better/ more efficiently than a woofer with a FS of 45 hertz.
True, but this is blown way out of proportion... and people will forever think that an 18 can't keep up with a triple bass pedal.