I note that you are in the UK, like myself - I'd recommend a Lindy powered USB hub. (Don't know if Lindy kit is available worldwide.) It'll actively divert power from port to port if needed, or remain passive if not. It's nice and small too, and looks good (it's silver) with a Mac. Mine was £25 or so.
I'd also suggest plugging the Soundsticks into your Mac, rather than the hub, if at all possible as USB traffic may have an impact on sound quality.
You can buy direct from Lindy, but the site is down at the moment for improvements. You should be able to get them somewhere in a city the size of Brum. I love their stuff, and they are very helpful and knowledgeable if you ring them up.