Sean is correct, loads of brands throw the word "quality" around all the time, and the mass majority of the market doesn't take the time to really see if it's true.
In this case, quality of the subwoofer is about the performance, the fit and finish, the end sound, does it do as described, and the people behind it (i.e. warranty/CS). As for as sound, the Icon is, and has always leaned a little on the warmer side (thicker or more dense) of sound characteristics, due to the large copper coil and heavier cone body. The Hellion is a little more neutral/drier of a sound than the Icon by a little. That is just something you will pick up over time and countless different subs in different installs on different music.