Tim Page ensures that his fall will start off with bags of vitriolic hate mail as he chooses 25 opera recordings meant to “give a novice listener an opportunity to explore the field.” “The selection process was not easy. Operaphiles are an opinionated lot, and I can already anticipate some of the mail, both curious and furious, that I’ll receive. A few sample heresies: Not one of the more than 60 operatic works by Gaetano Donizetti made the final cut. Virgil Thomson’s “The Mother of Us All” is here, but George Gershwin’s “Porgy and Bess,” an infinitely more popular American opera, is not. Where are the works of Benjamin Britten? And how can such composers as Mozart, Verdi and Wagner be limited to two operas apiece?”