Not exactly directly related to the OP, but I figured I'd throw these links in here:
http://chipmusic.org/forums/topic/8110/ … r-busking/
http://chipmusic.org/forums/topic/2681/ … -speakers/

I came across a portable amp when Maru visited the UK a few years ago. I got a link from him for an amp module which was what they used though it was soldered and customised with casing over the module itself. It ran on 4 C or D Batteries and powered two average speaks which was loud enough for playing on the street.

I don't have a better picture of this I unfortunately lost the file though might have it stored somewhere. It looks like this.

How it looks with two Gameboys ran though a portable mixer and a portable amp

Though the amp module was only available to purchase in Japan and the site no longer exists. I went into an electronic store to talk about it and they said what I was looking for was called an Amp Module. I did some research and found some alternatives for a portable amp:-

Digitech  - 2 x 15 Watt RMS Portable Stereo Amplifier (8 AA Batteries) - http://www.jaycar.co.nz/productView.asp … TID=1009#4

This portable amp is stereo though only has one 3.5mm stereo input as opposed to two phono for input. It's 15watts and can be powered by 8 AA Batteries.

Does anyone else know about Portable amp modules like these? They're ideal for carrying around and light weight compared to car batteries or other amplification.