Most sources for the English audio were from Laserdisc, we have nice mixes now (not necessarily exactly the same as those used for the DeEd's) that use "best of breed" releases, with patched over dropouts, etc. So it's lossless in the sense that it's from Laserdisc, mostly digital audio, but occasionally patched over with analogue audio, originally 44.1k 16 bit lossless, resampled to 48k.
"Lossless" is always a bit of a dodgy term, because much could have been altered for the Laserdisc release (dynamics, channel separation, etc), but that's pretty much exactly as good as Blu-ray lossless in that sense--moreso in that they didn't routinely tweak the audio so much back then.
The Star Wars mono mix is mostly from a capture of a European TV broadcast, expertly cleaned up and synced, and doesn't really sound much worse than partial captures we've gotten directly from 35mm--academy mono really wasn't all that great, so it seems that's just about as good as it sounded originally. Empire 16mm mono mix is, surprise, from a 16mm film capture, and is pretty rough.
This thread is about one mix in particular, just the 6-channel 1977 70mm reconstruction, which is from multiple sources, all Laserdisc IIRC except that some of the LFE track is derived from the Blu-rays.
IIRC these 6-channel mixes for ESB and ROTJ are content-equivalent to "tasteful upmixes" of the original stereo mixes, not reconstructions of the original 6-channel mixes. ESB's theatrical 6-channel mix was entirely different, matched an entirely different 70mm cut of the video, and our best source is a pretty dodgy-sounding 8mm capture that's missing some big chunks--and we don't have a complete re-creation of that. ROTJ's theatrical 6-channel mix has no known differences from the stereo mix content-wise (but no reference either, so who knows), so a tasteful upmix of the stereo mix may very well be pretty close. ESB got some LFE from the Blu-rays in the latest iteration, ROTJ will probably get the same treatment once ROTJ DeEd 2.x is closer.
