Anna Thorvaldsdottir Enigma EP Spektral Quartet Clara Lyon, Maeve Feinberg, violin; Doyle Armbrust, viola; Russell Rolen, cello Sono Luminus CD, 2021   Enigma, the first string quartet by Icelandic composer Anna Thorvaldsdottir, arrives after a spate of engaging chamber works that have often featured strings, but never in this most traditional configuration. The piece has essentially two very different experiences to offer to the listener. The one considered here is a well engineered CD recording with detailed antiphony that gives a sense of the spatial dimensions of Enigma’s live incarnation, a multimedia work in a 360 degree full-dome theater space,