There, we were taught to cook Miso soup. Firstly, you're not supposed to add the Miso paste into the hot (below boiling point) Dashi stock. Only then, the supposedly biologically active Miso paste won't 'die' and thus ending the health benefits of it.
You're supposed to scoop the hot Dashi into another container and then add the Miso paste to dissolve them together.
Once dissolved, switch off the fire for the Dashi stock and add the dissolved Miso mixture, stir and then serve. When re-heating, do not bring to a boil because then the taste will change and you loose the "friendly bacteria" supposedly good for the flora of your intestines. Check out the last line of the extract below on Miso nutrition from Wikipedea (Lactobacillus acidophilus ).