Theory is fundamentally different from research. In particular, research involves collecting and analyzing data, neither of which occurs in theoretical work.
David is correct. What you are exploring is 'methodological'. It's what is needed before collecting and analysing data.
Empirical research is 'that which is observed' - and that may be from the perspective of researcher and/or participant. It can equally be both quantitative or qualitative. It doesn't matter if it is positivist or post-positivist. It also doesn't matter if it is theoretical or philosophical.
It is well explained by David and Dean, just to add mine. A theoretical research is the one which is hypothetical. In this type of research, the researcher does not use empirical data, for example interviews to build the theory, instead uses hypothetical examples. Hence, what it lacks is analysis of concrete data. Unlike to the theoretical one, empirical research is based on observed and measured phenomena and develops knowledge from real experience rather than from theory.
Thank you so much for the clarification, I have used them somewhat in succession. I now understand the main differentiator to be the type of data that is analysed i.e observed from say interviews which are empirical.