Hi, This wouldnt actually work to well to be honest. Firstly, one thing that many people dont understand, is that a subwoofer is meant to be placed on the ground, not on a desk. Placing a subwoofer on the ground will produce much more high definition bass compared to being on a table. So with it being in a laptop is not exactly ideal.

The other problem is that, a subwoofer has to be a large speaker - by convention:

..lower frequency sounds (bass sounds) travel through the air better in larger speakers...

and so having a subwoofer speaker small enough to fit i the laptop will not work, as you will not be able to sense any of the bass sound it produces.

There's a method of creating a psyco-acoustic impression of a low frequency by using beat-frequencies of two slightly different audiable frequencies. Listen to the sound in a twin prop aircraft when taxying as the engines change rpm slightly. Some software such as CoolEdit allows you to create beat frequencies as low as a few hertz (to simulate alpha waves for relaxation etc.) for example.The only problem is that you obviously hear the two normal frequencies too, so you'd need a pretty nifty DSP to figure how to modify the two normal signals in real time to fake a third, low frequence channel.