Opinions? Tune the box a tad bit higher...
There's very little program material in the 30hz range and I don't understand why you'd sacrifice efficiency for minimal SPL gains at lower freq. Tune above 35hz and use a subsonic. The only difference you'll hear is overall output; the 35+hz tuning will be a bit louder while it'll still hit the low stuff. Remember, a sub will play below the port tuning...not much, but it will.
The low sub-bass stuff is really bass you feel instead of hear. It's a lot easier to sneak a Bass Shaker under your seat and throw on a 40hz FMOD on a tiny 2ch than it is to install the cone area & power needed to reproduce those uber-low freq. with any affect.
My advice, really, is to plan on making a different box. I've never been really picky subwoofers as they only play 1-1.5 octaves of music anyway. A good box & a great amp will go a long way in making almost any subwoofer sound pretty good.