Some bassists hold down a song by laying an unbudging rhythmic foundation; others carry the tune aloft with intricate melodic lines. San Geronimo Valley native Owen Clapp likes to do both. “A bassist provides a bedrock for other musicians to build upon—it’s a supporting role, and I like being behind the scenes,” Mr. Clapp said. “But I’m also drawn to the melodic possibility of the instrument.” That duality will take center stage on March 7,…