The 1985 stereo mix is nearly identical to the original '77 in overall sound, with the obvious addition of the tractor beam line. The '85 version has a wider stereo image/greater channel separation.

The 1993 version is a stereo downmix of the 70mm soundtrack with additional effects mixed in. With its dedicated LFE tracks, the original most likely had greater bass content, but the '93 has as much as 16-bit 2-channel dynamics will allow for. The Star Destroyer flyby in the beginning has fairly strong content from 35-45 hz (the better your sound system, the more you'll get out of this), and other places have good response in the 50-60 hz range. As far as the added sound effects, Belbecus made a detailed comparison between the 93 and an in-theater tape recording of the 70mm, located in this thread. http://originaltrilogy.com/forum/topic.cfm/70mm-mix-to-93-mix-comparison/topic/6501/ While no mono dialogue was used, he notes that about half of the effects additions were also used in the mono mix(!).