Just a couple names that come to mind

Paradigm
Hsu Research
Velodyne

Peace,
Ben

I found a good deal on a Paradigm pdr-8 on audiogon. How big is your room?

for the price you're looking at you should be able to find some quality subs if you look used on audiogon and ebay.

oh and any of the above manufacturers should yield decent results...

I just went through this.   I was keeping my eye out for an adire rava which is supposedly the "giant killer" of the sub world, never heard it though.  Adire also sells DIY kits.  I was almost ready to buy a rava brand new, full price, since they never show up on audiogon or ebay, but then my local shop got a used Velodyne CT-10 which they sold me for an absolute steal.  I figure I'd buy this one now, and build one from adire later, after I educated myself a little more on subs.

http://www.adireaudio.com/

You can't really go wrong with any of the brands mentioned above though.  Take it easy - Matt

Tim,
Go big!  A good sub, even a big one in a small room will integrate seamlessly in with the rest of your equipment..  putting a big sub in a small room will actually pressurize the room..  which means you don't just hear, but you also feel the bass

Anyone have any experience with the Velodyne VX10?
It's only about $150.  10' driver, 150 watt amp.  
Touted as a beginner sub.  
I've been looking at the CHT10 (10' driver, 350 watt amp)....is it worth the extra $100-150 over the VX10?