Like those popular Enigma Variations of his, Elgar’s sprawling, wildly romantic Violin Concerto harbors its share of mysteries. Expect a majestic reading from Nikolaj Szeps-Znaider, who plays the very instrument on which the work was introduced. It’s known in the trade as the “Ex Kreisler,” honoring the fabled former owner Fritz Kreisler (1875-1962), the concerto’s original interpreter. And no, it’s not a Strad. The builder was Giovanni Guarneri del Gesù, widely acknowledged as Antonio Stradivari’s only rival. —M.G.