There's lots of things to look at. I think in one of those links there's some calculators to help figure out power efficiencies so you get the most from your speakers with what you have. The ultimate goal is efficiency within every variable that you choose to tackle. You can use WinISD...plug in your speaker parameters and it will help design a box using optimum air space. Every speaker is designed differently.
I purchased a 12" speaker designed to be most efficient in a .5 cubic foot space so that I wouldn't lose trunk space. So choose a speaker based on purpose and then match that with an amplifier. That should be the ordering process
