Why is a qtc of 0.60 with a vb of 3.0^3ft better than a qtc of 0.70 with a vb of 1.75^3ft????? May or may not be for you, depends on your goals. Every manufacturer has to pick a Qtc for their recommended box sizes. Normally they end up having someone in marketing choose a number that the engineers present to them that they feel corresponds to their specific market. Generically speaking .7 is considered normal and lower numbers are more damped higher more "warm". On top of that the Q of the box will affect the roll off in the frequency domain. In the car audio world this can also be chosen such that it utilizes the "average" cabin gain effectively. I think no one really answered because you said better or they don't understand Qtc. Different sure, better, maybe. For sealed boxes in a listening area I would definitely encourage polyfill, for a trunk of a car not so much. It isn't only used for making the driver think its in a larger enclosure.