Although encompassing a variety of research approaches, qualitative research in music education shares the assumption that reality is socially constructed; it takes this construction to be based on the specific perspective of the individual human; and it considers epistemology and ontology to be different fields of study. The posthuman theory of agential realism, on the other hand, argues that the world is becoming through intra-actions; it decentres the individual humanist subject; and it studies onto-epistemology -- practices of knowing-in-being. Considering these fundamental differences, the quality criteria of qualitative research are not applicable to posthuman music education research. Nevertheless, posthuman research is concerned with the ethics of research and how to response-ably and ethically take part in the world's becoming. Thus, the emerging field of posthuman music education research must develop other ways of evaluating research beyond the quality criteria found in qualitative methodologies. This paper argues that diffraction -- both as a physical and musical phenomenon and as a philosophical concept -- might be a fruitful approach. By reading quality criteria diffractively through the agential realist concept of response-ability, it poses critical and creative questions, moving us towards evaluating the quality of posthuman music education research on its own terms.