Experiment but I use the 4 ohm tap. The autoformer is an impedance matching device and most ML products are rated for 4 ohm. It dips down to .7ohm at 20khz. There is very little musical content at that frequency to cause a problem. I am using MC402 to drive ML Vantages in a much larger room in a mixed HT/2-channel setup. I will go deaf before the amplifier clips. You will be fine with 300wpc.
Also, you should place the ML speakers 3-4 feet from the wall or look into room treatments.