Image Dynamics, older JL Audio w3 and up series, DynAudio, Crescendo, DLS, Focal, Hertz, Audison, Morel, CDT, Seas, a/d/s, Audio Development etc etc. There are a lot of brands out there, each sounds different, costs different and will be a lot better than any Soundstream subs.
How much are you looking to spend?
How much power will you be feeding it?
What kind of enclosure are you going to use?
What amp will you be powering it off?
Why do you want something with a big magnet? Will you be installing it inverted?
There are so many questions to be asked and answered that any meaningful suggestions can be made. Go to
http://www.mobileelectronics.com.au and do some research and figure out what you want.