Max Pottag (1876-1970) certainly qualifies as a master hornist, performing for 40 seasons with the Chicago Symphony and having published pedagogical materials still in use today (likely the most widely used being the Preparatory Melodies to Solo Work).
The perspective he presented on tonguing is worth a closer look. In The Instrumentalist in their May-June, 1947 issue Pottag wrote that
Contrary to the opinion of many I believe it is better to place the tip of the tongue between the teeth instead of against them when starting a tone. The tongue is pulled back quickly as when saying the syllable “Ta” or “Tu,” and yet the action is similar to that when spitting away an imaginary piece of thread that has lodged on the lips. The between-the-lips attack produces a surer start of the tone than the back-of-the-teeth attack, especially is this true in the upper register of the instrument.
With the comment about with the back of the teeth attack he is going against the type of approach promoted by Farkas in his writings, mirroring instead the teaching of his teacher Friedrich Gumpert. A fellow Gumpert student, Anton Horner, presented the same approach as well in his few writings on the topic (more information in this article).
His statements are good ones to use to examine in comparison to the complete MRI Sarah Willis video or the more focused one that is only of Beethoven 3, both of which are below. What is the motion of the tongue? Is it “pulled back quickly” from between the teeth or does the tone start on the “back-of-the-teeth?” Is it sort of a combination of both, depending on the range or musical style? The videos give much to ponder. (Image source: YouTube).