Do you ever wonder what the "state of the art" euphonium player sounded like 119 years ago? Here is Simone Mantia with Sousa's band in 1902, playing his own composition Original Fantasy. What flabbergasts me is how the fast slurred pitches pop out so clearly. That is easier to get from a compensating horn, but to sound that way you need to align your airstream, chop setting, and fingers exactly on each of the 16ths/32nds. I've noticed the same thing from some trumpet players, like Harry James and Rafael Mendez among several others, who are obviously not playing on compensating pistons.
In college, my trumpet-playing roommate mentioned one of the trumpet players in the band he was in. He said it almost sounds like he is tonguing those slurred scales, but he was not. I suspect this is the kind of thing he was talking about.
This is from an old cylinder record.
https://youtu.be/WIwaHIJnVdI