If you had access to a time machine, what concerts would you go back to see?

Beethoven’s last Akademie (1808).

I have a friend who grew up in Detroit. He's about 10 years older than me, so he has regaled me with stories about some of the concerts that he saw at the legendary Grande Ballroom. I decided to do so research, and here are some of the shows that he either attended or could've seen:

10/14/67: Cream, MC5 and The Rationals
3/22/68: Eric Burdon & The Animals, Grateful Dead & Aere Apparent
3/29/68: The Fugs, Sly and The Family Stone & MC5.
4/28/68: Mothers of Invention &The Psychedelic Stooges 
6/1/68: Love, The Crazy World of Arthur Brown &  Psychedelic Stooges
6/9/68: Cream, James Gang & The Thyme
7/13/68: The Who, Pink Floyd &  The Psychedelic Stooges
7/19/68: Spirit, Fever Tree & The Soul Remains
11/24/68: Jefferson Airplane & Terry Reid
12/13/68: Deep Purple & Lee Michaels
12/27/68: Fleetwood Mac & Rotary Connection
1/12/69: Genesis & James Gang
2/1/69: Spirit, Jethro Tull & Sweet Wine
2/21/69: Paul Butterfield Blues Band, Van Morrison & The Attack
3/2/69: MC5, Three Dog Night & James Gang
4/11/69: The Velvet Underground, The Nice & Earth Opera
4/18/69: Chuck Berry, Brian Auger + The Trinity & Rare Earth
5/11/69: The Who, Joe Cocker + The Grease Band & Rush
This one is notable because it was the US debut performance of "Tommy." The Who's former road manager had taken a job at the Grande, and they wanted him to hear the first performance here in the states.
5/16/69: Led Zeppelin, Sun Ra & Golden Earring
11/7/69: The Kinks & Elton John

So, if any of this has gotten you the least bit intrigued, I suggest you check out either the documentary DVD "Louder Than Love - The Grande Ballroom Story" or the book "The Grande Ballroom - Detroit's Rock Palace." Both are currently available on Amazon for $18 & $22 respectively. 

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Every Dead show from 1968-71.

In the order of desirability:

Elvis anytime, Aloha from Hawaii if I get to pick.

An der schönen, blauen Donau, February 15, 1867

Lou Reed Rock’n’Roll Animal concerts at the Academy of Music.

Pink Floyd - Pulse
20 October 1994

I was almost 4 but at an early age I developed a love for Pink Floyd. I began to follow them from city to city at the age of 5 years only realizing that this masterpiece had been 1 year prior. What a shame.
But seriously - I would go back as my adult self and see this concert. My light is still flashing.