I was looking through all questions concerning the usage of そうです。. Now, in Course 4 Lesson 2 the way to use it is explained. However, only for い-Adjectives. Someone asked the question under the title "How do you use SOU DESU with -NA adjectives?", however the usage was only explained for the positive form.
しずかそうです。べんりそうです。... But what if I want to say that someones doesn't appears to be a な-Adjective? My guess would be using じゃない。 しずかじゃなそうです。 - Does not appear to be quiet. べんりじゃなそうです。 - Does not appear to be convenient