You have to understand that 35 years ago, arenas did not have the kind of PA systems that are standard fare now. If you look at pix of any band playing a large venu back then, you'll see a massive wall of amps and speakers ranged behind the band. The cable from the guitar, or bass, or keyboard went to a junction box that parallel-connected that cord to each of the amp inputs. It can get..... intense.

My Ampeg VT-22 puts out 120 Watts clean. Push it into distortion, and the numbers get downright scary. Johnny Winter used to play 6 of them at his blues concerts.

Look at pix of old Cream concerts, and you'll see that Jack Bruce and Eric Clapton each had two Marshall heads, each driving a pair of 4x12" cabs. Just try to find any mic or amp for Ginger Baker's drums. Back then, a drummer had to compete with multi-amp stacks, using just his physical strength, and huge sticks!

There are some things about the 'good old days' that I don't miss. It's a great feeling to stand in front of a wall of speakers that are so loud your clothing flaps in the breeze. But the horrible mix that often resulted, and the near-impossible conditions on stage, would not be acceptable today, nor should they be.