Thanks for the Valuable suggestion. I live in a apartment and Do you think a 500$ sub would make that huge of a difference? Also since my speakers are 100Hz - 30Khz, what frequency range should I look for in the Subwoofer?
Thanks
It depends on how much you value depth of bass versus quantity of bass. The $500 SVS subwoofer (some companies will sell you crap at $500) will give you a pretty flat and wide frequency response, and it will play as loud as you would likely want in an apartment (and maybe in your house, depending on the size of the room and your tastes).
As for the range for your speakers, the subwoofer should go up to 150 Hz or higher, as zieglj01 states. Most subwoofers can do that, but not all. For the bottom end, the lower the better. But, you should not select purely on frequency response, as how loud it can play without much distortion is also important. For all of this, I would go with the SVS at $500, and recommend waiting until you have that much, so you don't decide in a year or two that whatever cheap thing you bought isn't good enough.
I have had a cheap subwoofer before, and I don't think I made the right choice in getting it. This is what I would get for $500 (and nothing for less):
PB-1000
For more money, you can get higher volume levels (which particularly matter in large rooms) and somewhat deeper bass. But this one is very, very good, far better than what most people get, even if they spend twice as much in the typical store. This is a "factory direct" price, and they really make good subwoofers. Read the reviews, both at this site and at others. SVS is always a good choice.
(God, I sound like I work in their sales department. I have a couple of discontinued models from them, and they are the best subwoofers I have ever owned. And among the best I have ever heard. But I don't work for them and won't get anything from suggesting that you buy them.)