I'm late to the party (in more ways than one), but by the mid seventies all pro bands had monitors, good to excellent PAs (those lovely Martin bins that would just bludgeon you into submission) and their live gigs weren't any worse to what you hear today, just louder (max volume has been curtailed in Germany in the 90ies) and less taped vocals.
As for Grace - there were better singers out there, sure, but I always found her voice had a certain steelish quality which I liked. But what do I know, I even like Cher's voice. Grace was a piece of work on a bad day, but so were many male rock stars. She at least wasn't any worse than Lou Reed who's probably by now been kicked out of heaven for his mood swings.
And now for the medical part:
" ... whether sitting on your amp would damage your privates or not ... the jury is still out on that one ..."Lieber Nofi, this being the forum of scientific evidence, I can share with, console and at the same time reassure you: I have spent countless hours sitting on tube and transistor amps in too many rehearsal spaces to remember. I have two +20 year old children, they are physically and mentally healthy (the latter in a roundabout way at least).
I was at the urologist two weeks ago or so: He marvelled at the high blood oxygen and the fertility of my testicles (I have two, unlike other notable Germans in history). Any better, and the mere touch of my scrotum would cause fingertip touch induced pregnancies of the other - what am I talking: any!!!! - gender in osmosis-like fashion!
I'm glad I could help clear that one up.
PS: Actually, my favourite place to sit during a rehearsal is my 18" Ampeg cube cab (the 8x10" fridge is no good for that, I'm a little uncomfortable with heights). Hitting a low E is a nice, manhood-reassuring feeling on there.