Rubbra's Improvisation belongs to the vast library of works commissioned by the Louisville Orchestra from 1948 onward. The fantastic soloist is Sidney Harth, who was concertmaster in Louisville before moving on to Chicago and Los Angeles. Invariably I think also of Sidney's son Robert, who, until his tragically early death in 2004, presided over the greatest period in the modern history of Carnegie Hall. Also worth a listen: Wallingford Riegger's Variations.