Although I love the work in its original form as part of a string quartet, the string-orchestra version overwhelms me. The Adagio becomes for me two different pieces. As an independent work, its unashamed Romanticism cries out for big sonority, and The Sixteen are just too few. Again, Warland beats those I've heard. The difficulties of the piece are essentially those of the Allegri Miserere - keeping pitch ranking as the chief obstacle.
Strings have less of a problem sustaining pitch while holding notes.
They also don't confront the problems of breath the way singers must, so the phrases tend to stretch out. I've heard plenty of choirs suddenly drop the musical thread. The Warland Singers' pitch is spot-on and the tone clear as a silver bell, while they summon up the necessary weight for the sound and length for the phrase. Even more important, the piece is essentially a series of builds and fallbacks, and for a choir, louder is usually easier than softer. Again, the Warland Singers give the impression that it's simply the most natural thing in the world.