Very pleased with my Velodyne
I have just recently purchased a Velodyne DLS-3750R, and am blown away with it. I recently had a passive sub with the Kenwood system I had, which was OK. I have recently went through a HT upgrade (To say the least) and I bought the Velodyne after auditioning many, many subs. They were all good (Polk, SVS, JBL, Mirage, Outlaw, Klipsch, etc).
I went with this particular Sub for several reasons. The price for one, which is right at your price point, obviously for the sound as well. And for one really important feature for me this sub has a remote control, for volume, and it also has 4 different sound fields that make a very noticeable difference in the sound that comes out of the sub. You do not have to continue to tinker with it. And although everyone may not want to admit that they do, a lot of people will admit to having to constantly "Tinker" with their settings till they get them right.
Now which way do you want to set yours up? By having to constantly having to turn dials on the sub (Which a lot of the controls are behind the sub) or with a remote control? I do not mean anything negative about any other manufacturer of Subs but Velodyne is on to something here.
Also the DLS stands for "Distortion Limiting System" meaning that the sub has electronic circuitry in it that eliminates distortion. This thing weighs in @ 52 pounds and will deliver room shaking bass. Again besides the features what you want is quality sound, and this thing delivers in mass quantities, it really is amazing for what I paid.
There are a lot of other Subs out there, again most all of the really good quality, but I do not beleive any bring all of the features that the DLS-3750R brings for this price range.
Go and listen to all you can before you spend your hard earned dough though!!! I just wanted to give you another idea!!!