Alright, I don't know how much you know about box design, and frankly I'm no expert, but I'm gonna give you the rundown to the best of my knowledge. Subs have a resonant frequency. This is the frequency at which the speaker naturally vibrates in unison with the signal sent to it. I know it sounds dumb because a speaker has to vibrate at the frequency of the signal, but the resonant frequency is very pronounced, and around this frequency you can make a sub very loud with very little input. Like I said, I don't know how much you know about box tuning so here goes. Below about 100 hz the human ear can't locate sound, it can hear it but can't figure out where it's coming from. I think below 20 hz is subsonic. You can feel the pressure wave but can't actually hear it. 40 hz is used as a tuning goal in lots of applications where volume is the main objective because quite a few subs have a resonant frequency somewhere around there. The problem with tuning your box to the free air resonance of your sub is that when your system hits a note around that frequency, the bass gets loud, REAL loud. I used to run a DD9515 with a box tuned to 35 hz (the DD's resonance frequency is 33 hz I believe) and when it hit a note that was between 35 and 40 hz, the whole neighborhood knew it. Another problem with tuning a box to your subs resonance is that the sub tends to distort pitches right around there because it's throwing it's own input into the equation. So now you have loud and distorted bass at the point. I've never run Kicker's so I don't know how they sound, but for street driving, I'd try and aim a little higher for box's sweet spot. That way it'll be more in your usable range of the music you listen to. 40 hz is pretty low for average music to go. The only time I really ran into it was on test CD's and in really low rap music. I run a Memphis SHP 10" now and a friend built the box for me, but I believe it's tuned to like 50-55 hz and it sounds really good for what I listen to (mostly hard rock, but still bumps hard when I turn the rap on) It's still louder when I hit the sweet spot of the sub or the box, but it hardly distorts at all, and the volume spikes is true of any ported system.
So...If you don't feel like reading this long post, buying the box that's tuned to 38 hz will work if you want to point and laugh as friends beg for mercy. (ask me how I know) but if you're going for SQ I'd try to get a box that's tuned a little higher.
