^^ not really true. Prefab or not, if the enclosure is the correct size for the subs, they will perform as they should. As for the sealed aspect, that's more of personal taste. yes a ported will put out a little more, a little, but if you like the sound of a sealed enclosure, such as I, which is why I have my (2) 12's in a sealed enclosure and they hit hard and get nice and loud, then that's what you like. plus they are typically smaller, saving space and you can still do some minor tuning to them by adding poly fill or wood blocks to make the enclosure slightly larger/smaller, if need be.
I would suggest however finding a better/bigger amp. You will be better off getting a bigger amp than the rating of the sub. you'll be clipping/distorting before you'd ever be able to hit the full, supposed, 500w that amp is rated to put out. plus if you get a bigger amp, you can run it lower, putting less stress on the amp and letting it run cooler.