1)The larger the driver, the lower the sound of the bass output.
2)A speakers that reaches below 30Hz at its low frequency is typically great for movies and games, as it will be able to hit the low-frequency effects. For music, bass players usually stay in the 40-50Hz range.
3)If you can wait for sale, the PSW110 or PSW505 will be good fit for you. I have one PSW505 I got it from newegg openbox for less and I love it.
Here is review http://bestsubwooferreviews.org/polk-audio/ http://www.hometheaterhifi.com/volume_12_2/polk-psw-505-subwoofer-4-2005.html