I've got a DIY sub that I built a few years back using one of the Parts Express 500W plate amps and a 15" Adire Tempest driver. The performance is very good, but as Haoleb noted you have to build the enclosure to the specific driver you're going to use. Just because it's the same size as an EP500 driver does not mean that's the correct or optimum enclosure. If you can find the driver manufacturer's website, they might have some suggested enclosure designs. Otherwise, use a speaker design program to come up with the right size and shape.
WinISD is one.
In order to get the best performance out of your sub, you really need to put some thought into it. The construction process is not particularly difficult, but you need to be very attentive to the dimensions and details. Sealed boxes tend to be smaller, play lower and are more accurate, while ported boxes play louder and can be specifically tuned to certain frequencies.
As far as the Parts Express plate amps, I think they are a great value. You can save a lot of money building your own sub. I'm very happy with mine, and in fact, I prefer it over my EP350v2.
