The Twin Reverb behind Billy is a guitar extension (maybe just Jerry, since Bob's column is right there?) , and I'm not sure about the 2x12 on top of that... piano maybe?
Phil is out of position, yes, but still standing directly in front of one of his two massive towers, and apparently LOVING it!
"There, in huge black letters, was 'The Grateful Dead'. It just... cancelled my mind out."-Garcia