I was at a Tennstedt M6 in London in 1983 at the Royal Festival Hall. I remember a huge triangular black Maria box at the back of the orchestra that delivered terrifying hammer blows in the last movement. The intensity of the performance was overwhelming. I thought we were all going to die, including Tennstedt. My wife said it was the most powerful performance she had ever heard, but she didn't want hear the symphony again for at least ten years.
We may remember the scene of Mahler weeping after conducting the 6th.