The big summit meeting for music fans in the Boston area this week is the Dave Alvin and Jimmie Dale Gilmore "Downey to Lubbock" tour, which played the Narrows Center for the Arts Saturday night, and touches down at City Winery in Boston on Sunday night.Fans of roots music no doubt know both men, but many listeners might view it as an odd pairing, even if both are indisputably pillars of the wide-ranging Americana genre.