With their sugary riffs and shout-along choruses, CD9 initially seemed like Mexico’s answer to One Direction, bringing a dose of brash rock attitude to the classic boy band template. But on their latest EP, the quintet suddenly embraces the languid pulse of tropical pop. The banging drums of reggaeton-inspired slow jams like “No Le Hablen de Amor” show that CD9 can now groove alongside the grownups, but the infectious “Rompecabezas” is proof that the group hasn’t strayed too far from its bubblegum-pop beginnings.