When the weather is nice and you decide to take a sunny stroll down Market, keep your ears open for Moses Dixon, the 62-year-old vibraphone player who frequently busks on the corner of Grant and Market on balmy afternoons. Dixon sets up his vibes outside the Wells Fargo building and jams for tips. And he deserves big ones. A tape of prerecorded, hard-swinging bop blares from his tinny boombox as accompaniment while his mallets sprint up and down the keys. But it only makes sense as jazz when you get up close. From blocks away, Dixon's improvisations create a much more surreal effect. The metallic clang of his instrument bounces and echoes off the soaring facades of the Financial District in a harmonious blur, coupling with the hum of passing cars and the distant wail of sirens to create a wondrous soundtrack to this corner of the city.