Lead singers serve as a visual and aural focal point for their bands, but they can also be a total pain in the ass when they have successful solo careers, split acrimoniously from their band mates or die prematurely. So it's fitting that in a touring season filled with replacement singers fronting classic-rock acts (such as Styx, Boston and Journey), long-time survivors Chicago and the Doobie Brothers will share the stage. Chicago has always rotated lead vocalists, but the honey-throated Peter Cetera helmed most of their biggest hits of the '70s and early '80s. Cetera is long gone, but bassist Jason Scheff has kept the dream alive since 1985. Likewise, the Doobie Brothers have long labored without St. Louisan Michael McDonald's inimitable pipes, but the remaining Doobies plug the gaps admirably. Truth is, these AM Gold chestnuts sound pretty good from the lawn at Verizon Wireless Amphitheater, no matter who is singing them.