By a quirk of summer concert scheduling, Houston will get to sample both periods on the same long weekend. The current Doobies with Johnston and Simmons (who reformed the band in 1989) and near-original drummer Michael Hossack headline the BP Power of Freedom Festival. While they'll rely heavily on the party power classics with a potent and energetic stage show, they'll also probably mix in some newer material ("The Doctor" and a couple of tracks from their latest, Sibling Rivalry).

McDonald -- whose post-Doobies career saw decent '80s solo success with more MOR material ("Sweet Freedom," "I Keep Forgetting") -- by contrast comes with two bands: his own, and a little backing ensemble called the Houston Symphony Orchestra. Interestingly, according to his Web site, Houston is not only the first date of the tour, but the only one with such classical leanings. Expect a career retrospective, along with a sneak at his most recent project: a batch of Motown covers due out later this year.

For a listening primer, the recently released (and long overdue) Greatest Hits brings the best of all Doobie eras together on one disc. But with only one day separating the shows, the questions hangs: Will either stage feature a surprise "family reunion"? Bootleggers, start your tapes…