Anyone is free to disagree with me but they'd be wrong
(just kidding) - more seriously, I think it is important to at least watch pure sine wave behavior and how clipping/shaping them creates harmonics, just sayin' I don't care what the ask is in that regard because it's something worth understanding first and foremost.
I picked sine because it is a basic building block with zero harmonics, and if he cares about what preamps impart, the first thing he should do is run a sine through one, turn it up until he just starts seeing harmonics, then decide when he actually starts hearing them and so on. I'm seriously considering doing a YT tutorial on sine waves all the way down to strapping a pencil to a transducer to demonstrate how it can draw a sine wave coming from a DAW.