In different countries, the relative prominence of Marx and the Critical Theorists differs, I think. Where I have lived, Marx is a topic of study in, say, university classes for freshman. Nobody hears about Adorno and Habermas.

I do agree that any reading of the Frankfurt School philosophers without first covering Marx is impoverished, because they are explicitly trying to generalize the transformative approach of Marx. For instance, Marcuse's early works are particularly focused on the same issues as Marx was, and his later ideas of "radical subjectivity" parallel Marx's ideas that revolution is necessary. Horkheimer's texts are full of language about needing to support the proletariat against the bourgeois.

But where I've lived, nobody outside weird academic circles really has heard of critical theorists, let alone read their works, even if their ideas and perspectives are shaped by them; but quite a few people have exposure to Marx (though to be fair, tons of people also have very strong opinions on Marx based upon hearsay and/or cultural "wisdom" and no direct exposure to Marx at all).