They're bigger than a normal 15, so they need a larger box. I've heard 4.5-5ft^3 with a 30-32hz tuning works great, from a couple of people that own them.
4.82 cubic ft, 15.5x6.25 port area, 40"long. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif should work very well.