Being a van causes a few issues. 1, the size of the interior and 2, the sides of bands and minibuses tend to be quite flexible. Sub bass in a car relies to some degree in the cabin accoustics to amplify the sound. There are three types of enclosure, a ported or vented, this trend to provide the really deep rumbling bass, the type that you feel as much as hear. A sealed box will provide a faster punchier bass for more of the punch in the stomach, and then finally you have infinite baffle (or free air). I don't know much about IB other than a lot of competitors are using them now, so I spent they can achieve both the benefits of sealed and ported but probably require specific subs and better tuning. To make the most of a vented box in a van you are going to need a big one with big subs and a big amp. I would suggest a sealed box somewhere up front with a pair of 8 inch or 10inch subs with about 250 to 350 Watts RMS to each one. That will give you some nice solid bass extension without sounding out of control or boomy. Read up on using twin subs at how to use 2ohm or 8ohm subs (and dual voice coil subs) in parallel or series to run of a single amplifier or a dual channel. Never load an amplifier with less than is rated ohms, and make sure when looking at Watts ratings you focus on RMS, there are some wouldn't ridiculous claims about maximum wattage. Also the cast audio manufacturers will give you some good subs, but you will pay a premium. Unless you are spending mega bucks, Companies like Scanspeak, Dayton audio, peerles/vifa make equally good subs for much better value. One last thing, whatever you get, learn to set the gain on your amp properly, many people think it's another volume control and them wonder why there drivers meltdown.