Motorola amplified speakers work very well. I have one for my 250D in my Jeep. They are available from time to time on eBay and since they're 12v you don't have to do too much to them to install them in a vehicle. Snip, strip, solder. Put a 25mm miniplug on the audio leads and you're set.
At home I have a Fostex 6301B amplified speaker that gives me more than enough output. It's volume setting is generally kept at the 2 setting...
Both of these take audio from the headphone output on the 250D.