A friend just pointed me at his new amp: the sonic impact T-amp. Lots of reviews if you google. Here's an interesting write-up of the history:
http://www.forbes.com/business/globa...1003/026A.html

Available on amazon: http://www.amazon.com/gp/product/B00009W44B/

$30 and actually very good so it's astonishing value. It's also tiny - under 10 ounces! - and can run on batteries (though performance is better with a decent main adaptor).

You can then plug in normal hifi speakers. So for travel purposes any recommended monitor speakers or quality replacement speakers for a midi hifi should work nicely and you get sound better than most cheap hifis. I hope to look into what specific good speakers there are but I've been web surfing for too long - back to work!

FYI: My friend is happy with the speakers he's used: JBL Control 1 speakers which weigh 1.8Kg (4 lbs).