For more than 40 years, the Kronos Quartet has provided the answer to the question, “I don’t know much about classical music—where should I start?” Unlike many traditional chamber music outfits, the Kronos crew—currently longtime mainstays David Harrington (violin), John Sherba (violin) and Hank Dutt (viola), with more recent addition Sunny Yang (cello)—is determinedly progressive and daring in its outlook, always seeking a new means of expression and eager to collaborate. They’ve covered songs by Dylan and Hendrix, performed with Bowie, Tom Waits and McCartney (not to mention recorded with Dave Matthews Band) and see genre as a mere obstacle to be pushed aside. In 2014, the group joined with four of its Nonesuch Records compatriots—Sam Amidon, Olivia Chaney, Rhiannon Giddens and Natalie Merchant—for concerts in London and New York celebrating the label’s 50th anniversary, then gathered in a studio to recreate the music. Each guest receives lead billing on two tracks, with Kronos keeping “Last Kind Words” to itself. That one is more age-old, back-porch string band than modern string quartet, and follows an eerie, hypnotic “Factory Girl” (a reworked traditional Irish number, not the Stones song) that finds Giddens’ delivery, as always, note-perfect and moving. Merchant’s “Johnny Has Gone for a Soldier” is tender and lush, unlike anything she’s cut on her own and, like Chaney on her two tracks, operatic in reach. Amidon, on both of his tracks, “Oh Where” and “I See the Sign,” achieves a balance between folk tradition and soul, but it’s Giddens’ finale, “Lullaby,” that most successfully demonstrates the inherent experimental nature of the hosts and their zeal to constantly evolve in untold, unanticipated ways.