“I ain’t really happy,” sings Gary Allan on “Get Off on the Pain,” the down-home masterstroke that begins and provides the name for his new MCA Nashville collection, “until the sky starts driving rain.” Unhesitatingly frank, mercilessly guitar-crazed, it’s the rocked-out country confession of a smart guy drawn to what the rest of the world calls wrong roads and long shots, or complains of as aching bones and stubbornness, or — as Allan sings in a spectacular stretch of drawn-out soulful vowels — underestimates as dark horses. And as the California-born superstar releases his eighth studio album, it’s about the most Gary Allan piece anyone could imagine.