Coxy: the speakers and amp as-is are only compatible with the USB outs on certain Macs, contemporary to the Cube and Digital Audio G4 (the DA designation in fact ~only~ means that they are compatible with these speakers). [NB: see correction in next post] I believe Griffin make an amp/adapter to use them with a standard audio output.

Your other option would be cutting and stripping the leads to the balls and wiring them directly to a suitable amp. A multimeter will tell you their nominal resistance - just find a small amp to match. One of those nifty Tripath chipset digital amps would perhaps be ideal. They are small, cheap, low powered and deadly accurate from what I've been reading.