I believe it has been established by some of the users here (like hairy_hen) going through all the home video mixes and comparing them that ESB and ROTJ use the same mix on all pre-1993 releases, which is assumed to be their theatrical mixes. I think there was also some debate about whether the 1993 ROTJ is a different mix or not, but that the consensus ended up being that it was a new mix.
I could be mistaken, a lot of that happened while I was gone and just lurked occasionally.