It turned out that my amp works off AC power internally. I just bought a DC/AC power inverter (usually powers from car cigarette lighter) and it worked fine.
I realize it's not computer related but the same principle could be used in making a powerful portable audio system for PC.
I'm trying to power my 40W bass amp by a 12v motorcycle battery. I already successfully powered 15W multimedia speakers this way but I'm having trouble with the amp...