I stumbled over this recording, https://www.youtube.com/watch?v=D0yajlp-JZg apparently from 1968, of TM students asking Maharishi questions regarding mantra refinement, how one knows whether one went "deep", and how to achieve "deep" experiences during meditation. I found it quite interesting (and a bit relieving and frustrating at the same time) that students' questions and concerns and the teacher's answers essentially haven't changed over all that time (especially also given the difference in transmission method between TM and DM).