The opening track of the Perfect Strangers’ self-titled 2003 debut, “Sing Me a Song (That’ll Just Keep Me Lonesome),” grafts dolorous lyrics to a buoyant waltz cadence–a juxtaposition that shows just how well this quintet of veterans, led by violinist Chris Brashear, understands bluegrass. Even the most mournful bluegrass was intended as party music, and Brashear’s jubilant fiddle patterns sway and spin around Forrest Rose’s propulsive bass lines. The rest of the players–guitarist Peter McLaughlin, mandolinist Jody Stecher, and banjoist Bob Black–join in on vocal harmonies that favor a sweet, rich sound rather than the jagged, shrill tone that characterizes so much bluegrass singing. But that sonority is offset by lyrics dark enough to qualify as Calvinist morality tales, portraying a world rent by catastrophe (Utah Phillips’s “Scofield Mine Disaster”) and isolation (“The Hermit Miner”), where men struggle under burdens that break both body and spirit (“Roll On, John”). Bleakest of all is their take on Bob Miller’s “Twenty-One Years,” a tale of crime, betrayal, and imprisonment–set, yet again, to a lilting waltz cadence. Marshall Wilborn will play bass at this show. Long Journey Home opens. Sun 8/21, 7 PM, Old Town School of Folk Music, 4544 N. Lincoln, 773-728-6000 or 866-468-3401, $20, $16 seniors and kids. All ages.