Re: so. why do you guys "swear" by custom boxes?
It's simple, different speakers and different application goals can call for vastly different enclosures. The downsides of a prefab would really depend on what type of enclosure your buying. For example if your talking about a sealed enclosure and, say, a 12" sub that calls for 1-1.25ft^3, that's a pretty standard size and shouldn't be too hard to find a prefab of. A more complex enclosure, like a bass reflex enclosure, could get a lot trickier to find a well suited prefab for your particular sub. The only other considerations i can think of is build quality and the materials that the manufacturer uses (for example a cheap manufacturer might skimp on mdf and use 1/2" rather than 3/4").
“Ignorance more frequently begets confidence than does knowledge" Charles Darwin