Janet Klein and Her Parlor Boys have been playing 1920s jazz-age music long enough to discover a nearly irresistible ‘prescription for joy’ and they’re bringing it to Marta Becket’s Amargosa Opera House stage this Saturday night. Headlining for the 52nd anniversary celebration of the famous theater-in-the-middle-of-nowhere, Klein’s ensemble tops off a program packed with good old-fashioned vaudevillian fun.