@petaluman 1++. This is very important for the design of sealed enclosure subwoofers. For the reasons petalumen explains, ported subs are inherently very poor performers. As was also mentioned the Qtc of a sub can be easily adjusted by changing the volume of the enclosure. With the advent of digital speaker control and the availability of high powered amps, I want the best transient response I can get given the available power so I shoot for a Qtc of .6. With speaker control and power you can easily force a good large driver down to 15 Hz in room.
Speaker control (room control) can also take the "hump" out of small loudspeakers and provide crossovers so you can integrate them nicely with subwoofers. Frankly, the only way to mate subwoofers with any loudspeaker at the state of the art is with digital signal processing.