"Ruth doesn't know I am going to say this," said Thomas Hampson from the stage of Herbst Theater Monday night, "but it's OK because I will cover the cost: if you feel you're not getting what you paid for, turn in your tickets at the end of the concert and your money will be refunded." Then, in response to some nervous titters in the audience, Hampson added: "I mean it, I am serious." The reference was to Ruth Felt, founder-director of San Francisco Performances, home to an annual appearance by Hampson for the the past quarter century. By sheer coincidence, I asked Felt before the concert if the unusually late and intense allergy season is having a toll on "her singers," and was surprised when she said: "Wait for Tom's announcement." Allergies (or "something," he said) got to Hampson already two nights ago, when he had to shorten his "Song of America" concert in San Jose. Now, before the beginning of a sold-out lieder recital, he had to make a decision whether to go on or cancel. He chose to perform, adding a money-back guarantee. Then, somewhat incongruously, Hampson used up a great deal of his remaining voice with a 17-minute lecture on Schumann, Schubert, Heine ("without whom there might not have been a Sigmund Freud," an aside to ponder), and - especially - the validity of his version and interpretation of Schumann's "Dichterliebe." To read about the subject, Hampson suggested his http://hampsong.com/ (don't bother with Hampson.com, that's the wrong address). This is a matter of passion for Hampson, to differentiate between Schumann's original 1840 "20 Lieder und Gesange aus dem Lyrischen Intermezzo," and the 1844 shortened version that goes under the (Schumann-approved) name of "Dichterliebe." To husband his voice, Hampson sang "only" a substantial cycle of Schubert Heine songs from "Schwanengesang," and those "20 Songs" by Schumann. With Wolfram Rieger's self-effacing and gloriously effective accompaniment, Hampson sang well, the voice ranging from a bit worn to as good as ever. At the end of the evening, no one appeared to have taken Hampson up on his money-back offer. Janos Gereben/SF www.sfcv.org [log in to unmask]