“Three of London’s major theater institutions – the Royal National Theatre, the Donmar Warehouse and the Almeida Theatre in Islington – acquired new artistic directors in the past year. A fourth company, the Old Vic, has a new head as well, a starry American one: Kevin Spacey, who plans to act in and direct Old Vic productions.” But can any amount of new blood manage to pull the West End out of what has become an embarrassing and extended slump? Michael Phillips sees some promising signs.