Zac Brown Band, "Jekyll + Hyde" (Republic)

Ever since 2008, when the Zac Brown Band made its first overtures toward the country mainstream, it has stealthily tried to remake the genre from within. Extremely facile, with tendencies somewhere between bar band and jam band, it was able to pass for conventional country on early hits like "Chicken Fried" and "Toes." Live concerts revealed something else entirely: a formidable, flexible country-rock outfit in search of bigger stages.

But as those stages became larger, the Zac Brown Band became less interesting. Brown is, at best, a meaty, un-nuanced singer, and the less complex the song, the fewer places he had to hide. Country has become more diverse and open-eared, but not necessarily in ways that benefit the Zac Brown Band.

The amiable "Jekyll + Hyde," its fourth major-label full-length album, suggests a path forward. Rather than follow the hip-hop hybrids of the day, the album offers a huge amalgam of soft rock, country-rock, hard rock, heavyish metal, big band music, bluegrass and, yes, a touch of electronic music.

"Jekyll + Hyde" is the sort of album that giddily puts a song featuring Sara Bareilles back to back with one featuring Chris Cornell, the kind of omnivorousness that went in and out of fashion in hip-hop more than a decade ago, but still feels novel in country. On the Bareilles song, "Mango Tree," Brown turns into lounge singer over an unremarkable big band arrangement. On the Cornell song, "Heavy Is the Head," Brown finds something approximating a rock growl, though not a tough one.On songs like "Loving You Easy" and "Young and Wild," Brown's most familiar mode might comfortably be called nu-Chesney, a revisiting of the beach-friendly sounds that qualified as radical a decade ago.

Those modes serve him well. His unimaginative voice can gum up a song, as it does here on "Dress Blues," and he rarely moves past lyrical platitudes. When he evokes Kenny Rogers, as on "One Day," it's effective, but more often it's a liability. As in the past, he's best when excavating deep feelings. "Bittersweet" is about learning a loved one is about to die, and facing the impending tragedy with an open heart.

Even when Brown is taking it easy, though, the band is working hard. Take the Celtic-ish blues of "Remedy" or "Tomorrow Never Comes," a lightly gothic electronic-music-inflected bluegrass song, and one of the album's most exciting.

But for every song that issues a challenge, there are two that play nice. And there are nods to mainstream country values, like "Dress Blues," about a young soldier who dies at war, which concludes with a messy violin rendition of taps.