Any list of the best two-voice harmonies in pop goes something like this: the Everly Brothers, Simon and Garfunkel, and the Indigo Girls. Now, favoring female voices as I do, I’d put the folk-pop Indigo Girls, Emily Saliers and Amy Ray, at the top of that list, but squabbling over details aside, it’s the seamless harmonies that make the Indigo Girls such a draw after such a long time. The recently released Poseidon and the Bitter Bug is their 11th studio album, and the bonus set includes an all-acoustic version that’s fabulous. But rest assured, the Indigo Girls have never been able to resist turning up the amp once in a while, so don’t expect an unplugged show. The set list is bound to be heavy with the new material, which won’t hurt since it’s up to par, and they’re bound to dip into the vault for some favorites, too.