Back when Jason Isbell was the number-three songwriter in Drive-By Truckers, wonderfully evocative songs like “Decoration Day” and “Danko/Manuel” stoked expectations for what the prodigiously talented singer-guitarist might do when finally given the chance to make his own record. But on his solo releases, 2007’s okay Sirens Of The Ditch and the new Jason Isbell And The 400 Unit, Isbell didn’t come up with a single song boasting the lyrical richness or emotional wallop of his limited DBT output. Isbell is an able tunesmith, so the gospel ballad “No Choice In The Matter” and the John Mayer-style blues-pop of “The Blue” go down easy and relatively painlessly. And Isbell clearly isn’t trying to emulate the grit of his former band; he’s opting for a radio-ready slickness that attempts to position him as a twangier Ryan Adams. It’s just that, based on Isbell’s track record, it’s hard not to have an empty, unsatisfied feeling when listening to the rote Americana-lite he’s now turning out. Isbell used to be good for two or three stone classics per DBT album; on his own, he’s yet another pleasant but ultimately generic singer-songwriter.