Brass sextets were the standard brass chamber ensemble until shortly after WWII; then the empire brass (quintet) changed all that and sextets fell totally out of favor.
But in the mid-1960’s there were still lots of sextets around, and publishers were still publishing music for them, so a bunch of my Renaissance arrangements got published as some of my earliest publications. Since then, all have been dropped by those publishers, but sextets are starting to make some comebacks, so these might be of use to you.
The exception is the J. S. Bach Ricercare a6, BVWV 1079 arrangement for brass sextet which is obviously a baroque piece.
Sextet arrangements are also useful for basic brass choir concerts; granted, all the instrument groups are playing the same parts, but it’s still a pretty massive sound. Several of the mp3’s of these sextets are recordings of a brass choir concert I put together in 1990.
All are for 2 Trumpets or flugelhorn, French Horn, Trombone, Euphonium, and Tuba.