For example, this recording from Osaka '96 has such a great sound quality that it's hard to believe it is actually an audience recording (what it obviously is, but still). As far as I'm aware of, regular cassette recorders are/were not capable of recording anything in this level of clarity and balance:

https://youtu.be/kz5T8NMGrBg

Even for a soundboard, the sound would be great. How is it possible?