Ok, I admit that I may have been wrong in my description of how to prepare proper miso soup. There are many different instructions available on the internet - they all agree on one thing and that is: do not boil the broth after the miso has been added. Some heat is needed, but too much will kill the beneficial bacteria.
Most recipes describe adding the miso after removing the water from the stove or after substantially lowering the heat (in which case I believe the temperature would be similar to that of water right out of a kettle). However, I did find one description which read that the correct temperature of the water before adding miso paste should be such that you can comfortably place your finger in it for 3-5 seconds (which is a much lower temperature than water straight out of a kettle).
So, I'm not sure which exact recipe to follow but if you want to be safe - perhaps follow the recipe with the lowest temperature. I think that's what I will do from now. Clearly I'm no expert and I've learned something new now - so thanks ozboz, and good luck!