First off, how big is your room? If you have a small to medium sized room, and you intend to listen to a lot of music through your system, you might also want to consider going with a sealed box sub.

The advantage of a sealed sub is that dropoff at the low end is more gradual than with a ported sub, they are easier to design properly, they are easier to place, and they have a quicker transient response. If you go DIY, sealed subs are also more forgiving than ported subs if your woodworking skills are less than precise. The disadvantages are that they are less efficient, the low end dropoff begins sooner, and they tend to distort more as they go towards the lower limits of the driver. Subjectively, some people say that sealed subs sound tighter or more musical.

The others have given you a good list of options to consider and they are mostly ported designs. I would add the following sealed subs to your list: the $400 Adire Audio Rava, the $450 Acoustic Visions MRS-10, the $600 Rocket UFW-10, the $600 Martin Logan Dynamo, and the sealed subs from B&W and Atlantic Technology. All of these options have slight differences in how they're configured, how big they are, and how they look.

http://www.adireaudio.com/TextPages/...eFrameText.htm
http://www.acoustic-visions.com/~aco...ed/mrs10.shtml
http://www.av123.com/products_produc...rs&product=1.1

Three years ago when I started shopping for subs, the only sealed sub in this price range I found was the Adire Rava, so I bought one. Now, the options have expanded tremendously. If you have a small room, the room gain will boost the low frequencies, and IMO this benefits sealed subs more because of their more gradual rolloff at the low end.

Regardless of which model you opt for, keep in mind that with subwoofers, your room acoustics are every bit as important as which sub model you opt for. This is also why IMO subwoofer comparisons are invalid unless they are all done in the same room using identical positioning and settings. Subwoofer listenings done in different rooms are pretty much useless because the room variation makes all the difference in what you hear, and the room effect gets progressively larger as you go into the lower frequencies.

The room acoustics are especially important if you have a small to medium sized room, because they will tend to develop more problem frequencies where you get huge boomy peaks and/or wave cancellations where the bass disappears. You can remedy this with careful placement, and by using room treatments such as bass traps and a parametric equalizer. Regardless, I think you should at least budget for a SPL meter and a test disc with low frequency tone on it, and a parametric EQ is highly recommended. Together, those items will run you about $170, but they are IMO a crucial investment if you're serious about getting the best possible bass performance. (Not just loud, but extended, and well integrated with the rest of your system)

I use a parametric EQ with my sub, and it made a huge difference in my overall bass performance. Out of the box, it was unbearably boomy, but with some fine tuning on the parametric EQ, the bass is now extended and subjectively tighter sounding because those peaks no longer dominate what I hear and measure. The chart below shows the before and after in-room response on my Rava (you can also see that the Rava has very capable bass all the way down to 25 Hz).