Need a battery powered amplifier

To make things short and to the point. I need a battery powered amplifier for a thing I'm doing where I'll have speakers strapped to myself playing music. Last time I used a powered phone speaker but it wasn't nearly loud enough. So this time I'm looking for an amp I can carry on my back as well and attach my bookshelf speakers to myself and play it with those. Does anyone have an recommendations for products that would be a solution?

Roland micro cube guitar amp would probably do the trick. Can run on 6 AA batteries. Mainly used for busking etc. You could output to additional speakers but the 5" speaker in the amp does just fine for what it is. Has aux in ~ phone, ipod etc.

Looking for just a stand alone amp since I don't have much money but I do have lots of speakers.

MY first thought is an O2. Might be a bit out of your price range. An E12 would work well too.