Santana / Journey / Eric Burdon @ Empire Pool 1975/1976 ish. Something wasn't earthed and the bands / crew seemed to care not a jot. Can still hear the hum 40 years later. Though that might be the tinnitus.
The Pretenders @ The Marquee early autumn 1979. Unnecessarily loud and sonically incoherent in a venue that usually made bands sound pretty good. They did look great though.
Psychedelic Furs / Wasted Youth @ Moonlight Club probably summer 1979 but possibly early 1980. Terrible sound, a great wall of dirty noise but on that occasion it didn't seem to matter as that howl seemed to be a big part of their shtick.
Rush @ O2 Arena a couple of years back. In essence a two hour drum solo though I swear there were a couple of other musicians singing and playing guitars out in the car park somewhere.