Hei hei all,
just following this thread now, great discussions & much appreciated, obviously I'm new,
It's already come out in parts, but I've been doing a few of these purchases through (so far) Naim Audio and Linn, and one of the other details that's responsible for the extraordinarily high sound value from some sellers is the whole recording process that they went through (Naim and Linn amongst many others I'm sure) - high performance in the whole process from placement of mics in recording, to all of the steps in handling/mixing/mastering etc., and that, I think, is why some people's FLACs are stunning and others sound worse than the commercially avail CDs.
One other note, I read a file that talked about someone doing a direct comparison (using the common ways of assessing the completeness of files in downloading/converting, md5 sums etc., not my specialty) and they found that taking a wav file and creating its flac, then converting back again gave exactly the same file based on those assessments.
I emailed HDtracks and asked about their "masters", and they said that they purchase masters from the actual recording companies, but I'm sure there are some other sites selling files that list who knows what source. Once can only hope to find a master recorded/mastered by a professional, and then it will likely be at least reasonable. Other than that . . .