Update 9/3/2012:

I'm planning to upgrade my subwoofer driver to help the deep bass output below 60hz or so. My current drivers are OK but I believe they can be beat in overall SQ and extended low frequency response without resorting to a ported enclosure once again.

The new drivers are on order (Scanspeak Discovery 12") and from what I have read these should offer a big improvement and shift the F3 frequency to about 37Hz or so down from est. 44 Hz. The plots look really nice down as low as 20Hz.

To help this effort I just read that the P99's SW network has some attractive High Pass frequency settings.
Currently I have my amplifier's infrasonic filter activated. It is preset to 30Hz, 24db slope. I use it because even a subwoofer with a sealed enclosure sounds better and tighter with it activated. I have tested this extensively.
However, to help increase the output of the lowest frequencies between 20 to 30Hz I plan to deactivate the infrasonic on the amp and use the P99 SW HPF to control the infrasonic.
I can choose either 20, 25 or 31 Hz and every slope is available from PASS to 36db.
Talk about versatility! Plus it will be nice to try this real time from the driver's seat with some reference deep bass tracks.
I have some older progressive rock that I love that has some deep bass organ passages. Really deep. I was kind of disappointed the last time I played it because I remember it sounding so much better with the ported enclosure. However I am NOT going back to ported. The SQ of the sealed is too great of a trade off to lose again. Therefore the upgrade in driver for a lower F3 as well as a little infrasonic / HPF experimentation with the trusty and versatile DEX P99.
Say tuned and I will report back when I install the new sub.