Opportunities to write for the alto trombone
I know some trombonists who just felt a disturbance in the force.
My brass ensemble has 6 treble instruments and 5 bass instruments, which can be assigned as 3 soprano instruments (or 1 soprano and 2 mezzo soprano), 3 alto instruments, 3 tenor instruments (or 2 tenor and one baritone) and 2 bass instruments. This information is really only useful as a preliminary way of getting my work organized. After, I look for ways to 'violate' these registrations that will add interest to an orchestration.
These rough classifications used as a baseline, can serve to understand some scoring exceptions that add spice to your orchestration:
extremes of the trumpet range
horn in the soprano or bass register
tenor trombone in the mezzo soprano or alto register
bass trombone in the alto, tenor (both sound different than a tenor playing the same line), or contrabass register
tuba in the tenor or (if you dare) alto register
These 'extreme' registrations can really make a musical line stand out, as long as you have the right people on the bus.
Regarding the arrangement above - as I have mentioned, we typically use 3 trombones and euphonium. Our virtuoso euphonium player is also a world class bass trombonist and this makes it possible for us to use a 4 trombone texture (opening up the Philip Jones / London Brass library to us). But this is one of the few times I have written specifically with a ATTB trombone section in mind - the trombone section here assumes the role of the 'chorus' from Bach's famous pastoral choral setting from Herz und Mund und Tat und Leben (Cantata 147).
To better facilitate the trombone choir, I lowered the key to E flat major - the choice of this key accomplished 2 things primarily: 1) the highest pitch of the chorale on a D flat (briefly) which is still reasonably achievable on a tenor trombone and, perhaps more importantly, keeps the trombone in a singing range; and 2) the highest note of the obbligato on a concert A flat. This consideration kept any part of the arrangement from sounding 'extreme.' We'll talk more about general key considerations later.
This is not paradigm shifting arranging, but it does illustrate the versatility and flexibility of the Boulder Brass instrumentation.