help with some info on walls

[DylanBunch]

So i have an x 18 and I want another so I can put both on my 3.5k and I want to wall behind my back seats. Now some things I have questions about.... 1) x series is a low end excelling sub, but I've heard if you tune low on walls its a lot more likely to fall of on the high durastically, any input on that? 2) what's a good rule of thumb on port per foot? Although all subs are different and that plays a major roll, do walls have different standards on port area/port per foot? My reasons for asking these questions are because right now its in a box @35hz and from 29-34hz it loses output cause its playing below tuning but then because its a low end sub, 28 and under it starts shaking stuff and somewhat moving air lol and about port area, right now that box is 16" port per foot and it carries down to about 20hz but on the high end it falls of at like 45hz and its a rather durastic fall off and my lowpass is set around 80hz on the headunit and amp. Thanks for input guys