Please clarify your question further. I think there is no theory applied to qualitative research, and one applied to quantitative research. The theory can be tested by any method
It's not clear what your question is really seeking, and there could be many possible answers, from ethnomethodology to philosophical hermeneutics to critical realism, to structural functionalism to semiotics to autoethnography. Are you drawn to any particular method? Do you have a particular population or sample in mind? Why are you drawn to qualitative methods? Are you interested in understanding experience and context, or explaining connections and causes? Is what you do with the findings a major factor? Anyone undertaking qualitative research would benefit from reading H. L. "Bud" Goodall's Writing Qualitative Inquiry, which covers the integration of theory into narrative via experience, crosses disciplines, and exemplifies good writing.
Qualitative and quantitative approaches are defined in terms of methodology. So, basically we can't discuss them in terms of either theory or practice insofar as we haven't defined a specific research area.
Considering your references to "social theory" and "posthumanistic research," I gather that you might be involved in fields as various as sociology, history of art, artistic production, cultural studies, etc. -- all of which connect in one way or another to philosophy in general. You might find it interesting if you get to have a look at Freud, Lacan, Foucault, Zizek and Deleuze. But before that, you need to narrow your scope down a little bit further and choose your object of study.