Today's version is done with red miso (in replacement of soy sauce). Miso (fermented soy) gives an added complexity to the otherwise "flatter" savory soy sauce.
Note: A simple technique to making a non-grainy bowl of miso soup is to slowly strain the miso over warm water. Hot boiling water might "kill" the good bacteria in miso. Adding miso-paste directly to boiling water will also be disastrously "grainy", sometimes "lumpy".