There was a great article in the Pro Sound News that talked about the Dead's move to IEM.
Apparently they were the first major act to do so.
I thought that the vocals cleared up a bit when they went to them...........
But, I also felt that "they" were in a different zone then "us"
It was early 90s,
if you look at pics, Jerry's 3x12 is not on the stage, it was under the stage, in another box, miked.
If you really want the inside skinny, find the pro sound news article.