most HT subs need big ported enclosures which is where this comes into play. If a certain speaker is not desinged for this it will lose its control in a large ported box when it was designed for a small sealed enclosure. BL plays a large factor in this in that the flatter the BL of a sub the more likely it can be used in a variety of enclosures and sound good at the same time because the motor is not too powerful for the mass of the sub or underpowered. Now lets take Ascendant audio for example, the avalanche in a HT application is meant to be in a 10-20cu/ft box tuned to 20hz to make it loud enough to fill the biggest of rooms and sound natural. I have a ava in my truck in a 2.5 sealed enclosure and it sounds extremly natural due to the fact that the size of my car is more relavent to the size of the box. If you were to take my 2.5 sealed enclosure and throw it in a ht application it would not have nearly as much authority as it does in my car. Now the reason some can work and some can't is because not all subs can be put in a variety of enclosures and still sound good and have control enough not to rech xmax at very low levels due to overpowering with little power. If you were to take a sub that is designed for a .75cu/ft sealed box with a rms power rating of 300 watts and throw it into a box 10cu ft sealed with 300 watts say good night to that sub, chances are it would be damaged with about 50 watts at continuous listening levels. Hope this helps a little.