I think the "desirable sound" for a euphonium was/is somewhat different in the UK/Europe compared to the U.S. I'm not as familiar with the UK/European sound legacy, so hopefully someone more knowledgeable can chime in regarding this history.
For the U.S., I believe the "American" sound concept started to evolve when the double belled euphoniums used by the US Military bands were replaced with the 3+1 compensating Boosey & Hawkes models imported from the UK in the 1960s. This change was spearheaded by Arthur Lehman of the US Marine Band. although his teacher Harold Brasch had started using the larger B & H euph a decade before).
This led to a darker sound becoming "standard" with US players compared to the lighter, brighter tone quality that was characteristic of the smaller bore, bell front instruments used previously, like what Leonard Falcone recorded his albums with, and players such as Simone Mantia used on the recordings you can find online. This legacy continued through players such as Dr. Brian Bowman (who taught many of the current and past US military band players), Earl Louder, Michael J. Colburn, among many. many others.
(this is what I can remember from my grad school exit exam studies. so if I messed up any of the information, please correct me!)
Edit: wanted to add that in general, there seemed to be a shift towards larger equipment (bigger, deeper mouthpieces, larger bells, bores, etc) in recent decades, which in turn can contribute to a darker, broader sound quality than what you would get with an older, smaller instrument. For one such example, a newer Besson Sovereign/Prestige compared to the Imperial/Round Stamps that preceded them. So I think the sound concept change is due to a mix of all three reasons you listed: instruments, players and the ensembles/settings they are used in.
Last edited by Fujiifilm; 11-16-2020 at 04:28 PM.
Reason: added some extra information
Willson 2900 TA-1 Euphonium - Denis Wick 4AM
Yamaha YSL-643 Trombone - Bob Reeves BrassArk 5G "Gladstone"
Yamaha YSL-8440 Trombone - Denis Wick 5BS
VMI 3301S BBb Tuba - Schilke Helleberg
Past:
York Preference 3067 Euphonium - Denis Wick 4AL
Benge 165F Trombone - Benge Marcellus
Wessex BR140 Baritone - Denis Wick 6BS
F.E. Olds Special Trombone (ca. 1941)