There can never be too much of a good thing as tonight’s show proves.  Bob and Bill were on for three, count ’em, three glorious hours of musical bliss, and every minute was put to good use.  Inspired by “20 Feet from Stardom,”  the Academy Award winner for best documentary, they paid tribute to female backup singers, whose work has long gone unacknowledged. And, in anticipation of their upcoming July performance, Bob conducted a mini-Blasters symposium.  Not a bad way to spend three hours.