And the power supplies have to be able to support the additional current needed. Power is proportional to the square of the voltage. Thus if the voltage is doubled into a given load, power is quadrupled. Power=watts, Watts =volts X amps, thus if the volts are doubled so is the current . There is where the problem lies. Think about sticking a 16 amp fuse where you stick the 8's now. Power supply has to be twice as big TO DELIVER THE SAME LOAD TO 4 oHMS, but an amps design in consumer applications isn't even designed to handle full rated music ouput because music is never a steady state, it's constantly going up and down so transformers are undersized relative to full continuous load.
In bridged mode I have no doubt the Macs can get around 400. Mac derated bridged operation to be able to advertise bridged mode into 2 ohms......