Forget about temperament! Freestyle tuning with the Tuning Card
Before the 1960s, harmonicas were in much better tune because it seems that making a perfectly-tuned instrument was the primary focus at the factory. (Little did they know the same things they were doing to the instrument to help make the chords play well also helped make it easy to bend notes!)
The Gapping Paradox
Gapping is adjusting the height of the tip of the reed at rest.
The height from the reed plate determines how much breath it will take to get the reed to play.