Arg! Stupid network fried my post. Grrrrrr. As I was trying to say...
The primary diadvantage of passives is the cost. If you can swing it, great! But often the radiators end up costing more than the drivers themselves, so people find more economical alternatives.
mjg does have some good advice though. EP is good, cheap power, and the MIC2200 gives you a subsonic filter and 2 PEQ's to play with. There's a huge number of 12" subs you can choose from, and he lists a few good ones.
Or, if you wanted a really cool build, you could use opposed drivers. Take it to the extreme and give your 13" cube a driver on each surface! That gives you about the same cone area as an 18" driver. There's plenty of inexpensive, quality 8" drivers out there, just check out Parts Express.
But definitely sit down and get modelling! There's no better way to understand the compromises you need to make, learning how the volume, cone size, xmax, and power all fit into the equation.