One thing that the couple could be talking about is the marked
improvement in background noise levels since the '60s. From the
microphones, to the cables, to boards and electronics, everything is
much quieter than in the '60s. They might be comparing
multi-microphoned Columbia recordings from the '60s to something
similar from the '90s. Because everything has a much lower noise
floor, they perceive this as an improvement in recording technique,
when the improvement has been in equipment and storage. All the
microphone techniques used today have been with us for a long time.
Because of modern materials, we can hear more detail (detail that an
audience member can't hear).