Herbert Howells never quite fully recovered from the death of his 9-year-old son Michael in 1935. The outpouring of music that helped the father deal with his crushing grief is among the most poignant and powerful of any composer in any era. Principal among them is a work penned intermittently between the 1930’s and 50’s, a requiem (though, like Brahms, the composer chose many of his own texts rather than setting the usual liturgical text). Showing the sketches of these intensely private works to Vaughan Williams in the 50’s, the latter persuaded him to publish them. St. Martin’s Chamber Choir presents the Howells Requiem along with Gerald Finzi’s haunting “Lo, the full, final sacrifice,” and a new work by Mack Wilberg called “The Prodigal,” both with organ.