I’ve just uploaded a new arrangement to Sheetmusicplus, a trombone quartet transcription of the Kyrie from ‘Missa Pro fidelibus defunctis‘ (by either Antoine de Févin or Antoine Divitis - nobody is sure!).
This is a lovely piece of Renaissance music suitable for players of all abilities, although the top part will require some stamina in the mid-high range. It is also suitable for euphonium quartet or other combinations of low brass, just get in touch if you require different parts/clefs.
I also highly recommend this incredible recording of the whole requiem from Marcel Pérès’ Ensemble Organum.