Miracles do repeat themselves ... sometimes. Reportedly, the heartwarming sweep of San Francisco Opera music director designate Nicola Luisotti's magnificent conducting became even more revelatory after opening night as he proceeded to push SFO's first cast (Gheorghiu, Beczala, Kelsey, Amsellem, Gradus) of Puccini's La Bohème to its limits. Faced with a new cast, however, and dealing with limitations that could easily be pressed beyond the breaking point, Luisotti chose to rein in his forces rather than overwhelm his singers. If the company's second cast had remained as announced, we would have experienced the long-awaited house debut of the much-heralded Maltese tenor Joseph Calleja (singing the role of Rodolfo), house debuts by Latvian soprano Maija Kovalevska (Mimi) and Brian Mulligan (Marcello), as well as role debuts by Adler Fellows Tamara Wapinsky (Musetta) and Kenneth Kellogg (Colline). Unfortunately, Calleja contracted a debilitating cold, necessitating his replacement at nearly the last minute by tenor Marius Brenciu, a Romanian. This created a major problem. Nothing could disguise the fact that our tenor was weak. Given Brenciu's credits — he won the 2001 Cardiff Singer of the World Competition; undertook the major roles of Nemorino, Edgardo, Lensky, Alfredo Germont, Count Almaviva, and Gabriele Adorno; and is preparing to make his Metropolitan debut as Prunier in La Rondine — listeners would reasonably expect him to possess a generous-size voice with a ringing top. He does not, at least he did not on opening night. The low and mid range was so lovely and healthy as to bring the great Tito Schipa to mind. But once the instrument approached the stave, let alone rose above it, it sounded pushed, blunt, somewhat unfocused, and out of its league. (The high C in "Che gelida manina" [Your tiny hand is frozen] was both underpowered and less than attractive.) And while the man may have an adorable, winning smile — I'd love to give him a big hug and pet him to death — most of the time he looks like an altar boy lost in the throng. With a tenor voice seemingly more suited to lieder than to opera, Luisotti in the pit had no choice but to either hold back the orchestra's emotional outpourings or upstage him and drown him out. Kovalevska must be one of the healthiest, least fragile Mimis on record. She turned her first-act coughing fit into a bedroom seduction scene, leaping on the mattress and attacking every note with the healthy relish of a teenager in heat. (Director Harvey Silverstein, where were you?) The voice may be strong and gorgeous — I'm sure her Mimi was heard throughout the cavernous Met — but its glamour in the higher registers speaks of divas such as Tosca rather than pathetic little creatures of the Mimi mold. There is no reason, of course, why anyone's Mimi should follow Angela Gheorghiu's lead and sing so softly as to become barely audible. But neither should a Mimi appear in the final act, as Kovalevska does, with newly applied lipstick, even pinker cheeks, and enough energy to pull herself all over the bed. Brenciu was quite moving when he discovered her lifeless; thank God she didn't rise up from the dead to sing an encore.