I definetly recommend a DIY sub. My
DIY subwoofer was a complete success and I spent less than $300 on it. I'm finally going to put veneer on it this month.
I used an
Ascendant Audio Atlas12" driver (discontinued) and I would recommend them in an instant.
With your budget you could built a real monster of a sub that would outperform virtually any commercial product available in the same price range. If you have the means to build a box, there is no better way to go!
A
Havoc 15" would make an amazing sub when installed in the proper enclosure and paired with a good amp. An
Alliance 18" would be an earthquake generator.
The people at AscendantAudio are very helpful. You can give them a call and they can make recommendations for drivers and enclosures based on what you want from your sub.