An artist's muse can be tricky to pin down. It's easy to recognize when a musician is in touch with it, drawing on some vibrant inner force. But what exactly causes that creative well to run dry? Fans of the singer and original Grupo Nostalgia band leader Luis Bofill have spent the better part of the past year or so pondering that very question. It was in no small part Bofill's soulful crooning -- as suited to a slow-burning ballad as to a growling, hip-grinding son workout -- that made Grupo Nostalgia's initial weekend residency at Little Havana's Café Nostalgia the local spot for serious fans of Latin music. And it's a credit to the rhythmic chops of all concerned that long after Bofill had settled into essentially going through the onstage motions, audiences still turned up to hear whatever new assemblage he was fronting. (Though that also speaks to many Miamians' desperate hunger for a Saturday-night alternative to clubland's canned beats.) But a funny thing happened on the way to Washed-up-ville: Without any advance fanfare, Bofill put together a new outfit, Cuba Libre (featuring several familiar faces), and adopted a decidedly new attitude. Just what precipitated this return to form is unclear, but as Cuba Libre's recent Friday-night performances at Coral Gables' Giacosa demonstrate, Bofill is back, and serving some of the hottest grooves around.