Not sure where you are located, however, there is a range of excellent subwoofers available here : http://www.mjacoustics.co.uk/

I use the 'Reference 100' model and I'm very happy with the performance. There are many UK dealers and a list is accessible from the link on the home page.

The sub woofer is connected to the line out sockets on your keyboard so does not cut out your internal speakers.
They do occasionally appear on eBay......

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Willum

[This message has been edited by Bill Norrie (edited 03-16-2006).]