I got my Wayback Machine out again and found this video from a Texas recital in 1986. I'm not a huge fan of Hartley's work, although I like some pieces and our euph/tuba quartet recorded a suite of his on our first album. Among his solo works, I was rather taken by "Sonata Euphonica" because it was a modern-style piece that seems to recognize a euphonium is named because it is pleasant sounding. Most of the piece is in mezzo dynamics (although he has a little bit of fun at the end with our service band role - listen closely or you'll miss it!) and there are a great many very lyrical melodic lines. No actual video, so it is in "slide show" format. BTW, I could not find another video of this online.
https://youtu.be/WE83s5XKZpc