A common cause of that in audio amps is blown output transistor(s), but could be something else. Is there any chance you can find a schematic or post good high-res pictures of the circuit board? Also look around the circuit board for evidence of overheating, or domed/vented capacitors.
I found what might be the schematic for it but don't have a chance to look at it right now, maybe someone else can make use of it.