Tuning a cabinet to extend response beyond & below resonance of the driver

@mijostyn +2 @mahgister +1 @erik_squires I have thoroughly benefited from the thread of two weeks ago about making adjustments to the Altec speakers. It was a fascinating read and I am impressed with all the sharing of knowledge. I have an 18" driver (normally I don't go beyond 15", but this has a BL 31, so I figured I had adequate cone control), and maintains a SPL above 96dB between 100Hz and its Fs 38, which is my relevant range for this sub. The curious thing is that its free-air response is flat enough that it stays above 93dB beyond its Fs and down to 25Hz, and I would like to build a cabinet that takes advantage of this, a cabinet that can extend its response from 38 to 25. Its Vas calls for a 4.13 cu.ft. cabinet, and its compliance is fairly stiff with a Cms of 0.05mm/N. With its Qts right at 0.40 I am undecided between a ported and a sealed. It moves a lot of air with a Vd at 1286 cc, and of course, that's without much excursion, so I feel it should have full, solid and tight, punchy bass. So, my question is two fold: how can I build a cabinet to drive the response down to 25Hz? And if so, does the cone stiffness hinder or help, and how can I best take advantage of the driver's properties? Are there resources or apps for me to consider that would help me? Any help would be appreciated!