i do not think qualitative research on its own is the answer.

perhaps you can increase validity by a mixed method approach. first, qualitative research to better understand inter-relationhips and to build a more comprehensive model, to in turn permit a better treatment of nuisance factors etc. hence, using qualitative research to improve the quantitative research

I must tell you it is not easy for me to interpret your question, though I am dealing with research methodology for a rather long time. It is not an issue associated with quantitative or qalitative research. I think the issue is: What sort of balance are you interested in or do you want to achieve and why? Following the action of justification of the logical properness of your proposed theory, internal validation and external validation are done with different objectives (biases vs. relevance), from different perspectives, and based on different measures. These are defined by the content and context of the concerned research cycle. The varying number of possible perspectives of internal validation and that of external validation may speak against any balance. I tend to believe that the question of equal strength of internal validation and external validation may not be a valid question in general, but may be relevant just in a specific context. Furthermore, it raises the issue of comparable measures. Internal validation and external validation together should make it sure that the proposed theory fit for purpose.