From that current requirements I would say you need to also include some driver transistors, MOSFETs or Darlington drivers as the Arduino can not supply that current by itself. Also you need an external PSU. The Arduino is rated at a maximum of 200mA, that means it can't supply more current than this, by itself it runs closer to 30 - 40mA.
The amplifier circuit is going to take more than 1mA. You can't use ohms law on this as you haven't got the resistance, in fact it doesn't strictly have one as it is an active device. But if it is a 2W amplifier then it must consume at least 2W of power and at 6V that means at least 333mA, given it probably only has 50% efficiency that would put it at 666mA. However I suspect it would be happy working on what ever voltage you put into the Vin on the Arduino.
Other than that I am not sure what you are asking.