The notes accompanying this recording are much given over to speculation about whether Mozart was directly influenced by the Requiems composed by his predecessors in Vienna, Johann Caspar Kerll and Johann Joseph F-u-x*, but let's consider them on their own merits.
Of the two, Kerll's Requiem of 1689 is a more intimate and personal work, written (as a note by the composer included in the published edition states) to be sung for his own soul and "also console others on my behalf". It is mostly for five voices (SATTB) tending to alternate between solo and tutti sections, with the accompaniment of a quartet of viols plus organ. The title is suffixed "c-u-m* seq. Dies Irae" perhaps suggesting that the Sequence was composed separately, and indeed it is in a different key and involves only four voices mostly as solos and some coro sections, with one duo and trio, and the instrumentation being rather more independent than the other sections where it is mostly doubling vocal lines.
In contrast that by F-u-x was composed in 1720 for the funeral of Eleonora widow of the late Emperor Leopold I (and was reused a number of times up to the exequies of Charles VI in 1740 whence it gained the soubriquet 'Kaiserrequiem'). Also for five voices (SSATB) it incorporates larger instrumental forces as the nature of the occasion would demand, violins, viola, violone, cornets, trombones, bassoon and positive organ. Structurally it alternates between sections with a small group of solo voices accompanied by strings, and a larger ensemble also incorporating cornets, trombones and bassoon. The Sequence Dies Irae much like that of Kerll is divided into sections alternating tutti passages with solo, duo or trio for solo voices and instrumentation reflecting the character of the passage.
This is my first purchase of a recording by the (nominally) Belgian ensemble Vox Luminis (whose personnel listed here include a number of German and Hungarian names). Each work employs two voices per part, with, as noted above, numerous sections with single voice per part. It's expertly performed, with pure expressive singing, in a fine acoustic location, and the vocalists are ably supported by the viol consort "L'Achéron" for the Kerll Requiem and ensemble "Scorpio Collectif" for F-u-x. After this I'll certainly be looking through the back catalogue of their works.
(*Necessary workarounds for Amazon's irritating and clueless "naughty word" filter).