Whoever said "the tape the lacquer was cut from" misspoke. As you point out, that tape is sometimes the original mix, and sometimes an EQ copy.
We always work from the original tape. In the case of the Decca and Reference titles, and some of the early jazz titles, that means the tapes that were run at the original session, just spliced together. In the case of something that was recorded multitrack, like the Robert Cray or The Band, that means the tape that was recording right off the console's mix bus during the mixdown.
Finding the original can sometimes be quite an adventure, but we don't go into production until we have found it. That happened, for instance, on our Staple Singers album. The vault first pulled up a safety copy. I actually took a couple of demo reels to the last CES made from the safety, just so I could give folks a taste of things to come. Then I carefully collected those demo reels after the show, since I knew the vault would eventually come up with the original mixes. They did, we mastered from them, and we are now in production on that tape.