Country legend Merle Haggard’s “Bluegrass” album isn’t any huge stylistic change for a man who’s always leaned on the hard country line and preferred Western Swing to any concessions to NashVegas. Hag’s been more of a purist than most and his traditionalism only becomes more pronounced as he soldiers on in a career that’s spanned five decades. The picking fits the genre, the songs lean towards Haggard’s own heroes (Jimmie Rodgers, the Delmore Brothers), his old catalog (“Big City”) and his own social commentary (“What Happened” shakes its head at an America overrun by foreign cars and WalMart). Marty Stuart joins on mandolin, Alison Krauss duets on “Mama’s Hungry Eyes,” and an all-star cast of Carl Jackson, Aubrey Haynie and Rob Ickes cast things in a relaxed, professional, but never overpolished, setting. Age has weathered Hag’s voice. No longer the same deep, resonant weapon, it’s become a weary talespinner’s yawp, conversational, saving itself for a few special notes. He remains an emotionally compelling singer, as the simple truths of “Learning to Live With Myself” resound with Haggard’s touching pathos.