“The John Legend of rockabilly.” And even, “the ‘what-took-you-so-long?’ fusion of Elvis Presley and Marvin Gaye.” At every performance, whether in a venerable metropolitan concert hall or a grimy dive in an anonymous strip mall, Dwayne Haggins is always at work. Adding new subtleties to an obvious melody line. Reinterpreting a time-worn lyrical cliché to imbue it anew with genuine feeling. Unleashing a gentle cascade of vocal acrobatics when least expected (and hence most effective) — never for show alone, but always to express a likewise unanticipated nuance of feeling: a whisper of love, a curse of anger, a groan of pain, a whoop of joy that feels, paradoxically, both surprising and inevitable at the same moment.