For the last six years the nearest thing to super-stupendous on the air has been the Lux Radio Theatre. Its casts have included all varieties of cinema hotshots. Its productions have often been so lavish that they overflowed the stage of CBS's Music Box Theatre in Hollywood. Even its rehearsals are a Hollywood event, with autograph seekers pounding at the doors. This week, after its usual summer pause, Lux Radio Theatre begins a new season with Myrna Loy and William Powell in the aerial version of Manhattan Melodrama.