For many opera lovers appalled by the excesses of regie theater, concert performances are an appealing alternative. Singers standing in sackcloth and rags are infinitely preferable to Hans Neuenfels’ whisker-twitching “Lohengrin” rats or Claus Guth rocketing Rodolfo into outer-space in a South Park Kenny space suit. The problem is that as Richard Strauss and Clemens Krauss wittily explained in “Capriccio,” opera has {…}