If you don't know the power consumption of the amplifier then the output power should be a third or less of the converter wattage rating. In your case, 320 watt ouptut should have a converter rated at least 1000 watts.
Ideally, any power product serving an amplifier (power conditioner, converter, transformer, etc.) should be rated at the volt-ampere of the circuit or 1,800 VA (watts).
Ideally, any power product serving an amplifier (power conditioner, converter, transformer, etc.) should be rated at the volt-ampere of the circuit or 1,800 VA (watts).