You LUCKY LUCKY man! I'd absolutely love to play a gig with a Who tribute band. You'll have a great time of it, their music is so energetic when played live. I suggest you take a look at some of their live videos on Youtube etc, from the Isle Of Wight Festival, Woodstock etc etc, that'll give you an idea of the pure power of the music you'll be playing.
Best of luck mate - enjoy every minute of it!
JEALOUS
Danny
PS - watch the drummer VERY closely!