Subwoofer recommendation

I am looking for a subwoofer to replace my existing one. Let me tell you about my system right now. I have 2 Fisher bookshelf-like speakers plugged into a small subwoofer i took off a karaoke machine. They are plugged in via the red and black connectors on the back of the unit. I then have the RCA cables leading out of the subwoofer, and into the jack on my computer. I have been noticing very little bass response when listening to the Fishers, and i was wondering if i bought a bigger subwoofer, i could hear and feel the bass when i listen to music or play games? Like i don't have a budget, i won't go higher than $120. I was looking at the Infinity TSS-SUB450C
Infinity TSS-SUB450C Charcoal 8" 100 Watt Powered Subwoofer at TigerDirect.com
What do you guys think?

Are the bookshelf speakers active or passive? Do they need to go through an external amp to be powered before going to a source?

Yea, they need to be plugged into an amp/receiver with those red/black wires to work.

The low priced Tannoy subs are good value.