Free air resonance

@mijostyn +2 @mahgister +1 @erik_squires This has been a fascinating read and I am impressed with all the sharing of knowledge. I just joined because (to me) I have an odd question and don't know how to start a new thread, and also because it ties into this thread. 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 of 96+dB 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'm torn 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, 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 take advantage of the driver's properties? Any help would be appreciated! 

@erik_squires I looked at DIYaudio, and I'll look again, but I didn't see content as well-directed as this thread. Is REM a cabinet simulator app? Are there any free apps that are adequate? I am wondering if my first question is even possible - to lower effective response with the cabinet? and if the stiffness will help or hinder in that regard? 

Thank you @mijostyn. Can anyone tell me if REM is the software that would help me? Or is there another app I should look into? And this is what I am undecided over: a ported closure to extend response from 38 down to 25, or if an infinite baffle will take it to 25 anyway without a port?  (I guess I need the cabinet simulator app as @eric_squires noted.)