Have you checked out hometheatershack.com or the diy section of audioholics. A lot of people there really seem to like Behringer amps like the A500 and the EP2500.
Yeah for about the same price as those plate amps you can get the EP2500 or 1500 and it will be far better. I originally had the Ep2500 for my subs but returned it a week later when I found a pair of used QSC RMX1450's locally for a great price. The only thing is you'll need a crossover as they dont have them built in like the plate amps. But if you then got something like the Behringer DCX2496 it would be far better and more versatile than any traditional sub amp.
Also, if you want to get any bit of performance out of your DIY sub you'll need to build the cabinet to match the driver's T/S parameters and what tuning frequency you want. Its not just as simple as building a box to put it in. People on the shack or audioholics or AVS can probably help you out with box specs.
