Check out Rythmik audio. I built a subwoofer based on their 350SE servo kit. Pretty impressive. I'd rank it up there with the big subwoofer from Rel. Also, Brain is very knowledgable and offers great support. They offer a lot of bang for the buck.
I purchased a plate amp from parts express when I built a sub for my wifes system to try and save a little money. There was a lot of hum, so I replaced it with a Rythmik 250SE; big difference and no more hum.
Rythmik Audio modifies stock plate amps, but how many companies do you see modding stock CD players or just basing their product on a stock design and adding a pretty faceplate.http://www.rythmikaudio.com/
If you really want to spend some money you can also look into Adire audio. I spoke to them breifly but they were out of my price range. You could spend upwards of $1000 on an amp and driver.http://www.adireaudio.com/
I would also recommend that you purchase some subwoofer modelling software. When I built my first, I did all the calculations manually and it was a chore. I picked up BassBox software, at parts express, and it made life really easy. It makes it so easy to change a parameter and see how it will effect the over all design.
I had so much fun building my first sub that I'm currently building my fifth and sixth sub now. I'm building a sub, based on the Rythmik 250SE, to replace the Kef sub in my office and I'm also redoing the wifes sub to make it smaller.