Headphones shouldn’t be flat on an FR graph like speakers because of the different way they interact with your ears. That’s why there isn’t a Harman curve for speakers. So if they sound like crap truly flat, that’s why. That being said, how do you know if it sounds flat? I’d say you’d have to have a trained ear and try to use songs with real unamplified acoustic instruments. If they sound natural, then it is probably close to flat. Not a huge fan of the sine wave technique because it isn’t real music so if you care about the music aspect rather than technical aspect I’d go with what I suggested above.