When you want to describe something as calm or subtle, the word "low-key" is perfect. However, if you're looking for some synonyms to mix it up, you might consider using "mellow," "laid-back," or "restrained." If you want to emphasize the understated nature of something, you could try using "subdued," "hushed," or "quiet." On the other hand, if you want to describe something that's intentionally low-key and understated for a specific purpose, you could use "modest," "unpretentious," or "unassuming." And if you're looking for a little slang, you might use "chill," "casual," or "easy." All of these synonyms convey a sense of subtle calmness without using the word "low-key".