A new recording of Tristan und Isolde with Pacido Domingo marks the end of an era. “No more spending hundreds of thousands of dollars coddling famous singers’ egos. No more waiting decades to, with luck, recoup an investment. No more recording works with singers who have never performed their roles onstage and are long past their primes. No more sessions spread out over weeks, months, years. No more relying on technology that permits a soprano and tenor to record a duet without ever setting foot in the studio at the same time. No more shifty studio magic. But then again, no more of that legerdemain to bring the music alive in ways it may never be in the opera house. This new “Tristan,” which EMI Classics will release in the U.S. in September, turns out — against all odds — to be glorious.”