Of course the other idea is to try and find a small portable battery powered amplifier - from what I remember from youthful days mucking about with electronics, they're not difficult to build - it might be worth popping into your local electronics shop (Radio Shack?), you can buy small plastic 'project boxes' to install it in. This would allow you to use ordinary computer speakers.
Richard
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Currently riding an MTB with a split personality - commuting, touring, riding for the sake of riding, on or off road :)