Infinity KP 12.1D sealed or ported?

In general, sealed enclosures provide better transient performance and driver control at lower frequencies, closer to an "optimum" Qtc of 0.707 and more musical, less colored overall frequency response as well as better power handling capability resulting in a more protected driver.  SPL and effeiency are not as good as in a bass reflex design, but ports can "boom" at the tuning frequency and cause overall SQ to suffer.

To determine for what kind of enclosure a driver is best suited, calculate the effective bandwidth product, or "EBP."  EBP = Fs/Qes.  A EBP of 90 or higher indicates the driver is best suited for a ported (or bandpass, etc.) enclosure, and a low EBP of 50 or less indicates the driver is designed for a sealed application.  Drivers that fall into the range between 50 and 90 can be used in either with certain trade-offs of enclosure size, frequency response, etc.  The Kappa Perfect 12.1 is a good candidate for a sealed enclosure because of its relatively low Fs at 23.66Hz and average Qes of 0.39, resulting in an EBP of 60.66, which is a lot closer to 50 than it is to 90.

The above should be considered the final answer.  "SPL or SQ?" shouldn't be an issue when you are seeking the best enclosure for any particular subwoofer.  "A combination of SQ and SPL" really means "loud SQ".  That shouldn't be too hard to get, once the SQ is achieved.  Whereas, if you were shooting for a SPL system, you would choose a sub more suitable for ported.