Andante Cantabile for Tuba/Euph Duet with Strings from Symphony No. 5 by P.I. Tchaikovsky for Euphonium & Tuba Duet with String Orchestra, arr. Forbes
One would not think it initially, but the warm timbres of euphonium and tuba duet works well in this gorgeous setting of one of Tchaikovsky's most memorable and popular melodies.