In a 5-quart pot, combine miso and gochujang and stir to form a cohesive paste. Add salt, garlic powder and sesame oil, mixing until fully incorporated. Add stock, 1 cup at a time, stirring thoroughly between each addition so the paste fully liquefies. When paste is fully dissolved, add 8 cups water and stir. Bring to a boil, cover and simmer on medium heat for 1 hour, or until soup reduces by roughly a quarter. Add tofu, cook for 15 minutes, then add clams and spinach. Once the clams have opened, discarding any that do not open, the soup is ready to serve.