It depends on what you call the master tape. You can have a) the original multitrack recording (often 24 track), b) the first generation stereo mix down master, c) a second generation distribution master copied from b) by the record company to send out to pressing plants or d) copies (or copies of copies...) 3/4/5/6th... generation thereafter. So called "safety masters" could be anything from a proper record company copy of a distribution master to a cheap poorly duplicated rip off from anything.
In days gone by, duplicating tapes was usually done accurately by record companies, accepting that one can lose a tiny bit of dynamic range on each duplication. Nowadays, there are fewer and fewer people, who can do this properly, with both the repro and recorder machines cleaned, demagged if need, lined up properly, repro azimuth checked on the master tape against the repair machine, and new tape bias set up correctly on the recorder. Ideally if you are copying, using Dolby SR helps to minimise any loss of S/N ratio in the new copy.
Stewart (Lodgesound) did indeed have a number of BBC masters, as well as some 1/2 inch stereo mic down masters from Sear Sounds in NYC. Audio Al is indeed correct in saying that they sounded fabulous in Stew's old house in Oxted.
Going back to the OP, a close generation tape will sound way better than even the most pristine first pressing LP. Mastering to CD involves further manipulation with compression etc, which can reduce the dynamic range significantly (hence the "loudness wars"). A well made digital master can sound every bit as good as proper analogue recording on multitrack tape. Like everything in life, it's not what you use, it's how you use it.
R2R: Studer A820 1/2 inch 2 track; Otari MTR-12 1/4 inch 2 track; Sony APR 5003; Sony APR 5002; Studer A807/II. Vinyl: Platine Verdier Allaerts MC1B/Schroeder Reference & Model 2 Decca C4E/Hadcock 228 TRON Seven Reference phono. Keith Monks MkII RCM Other analogue source: Nakamichi Dragon with ANT4066 mods. Amplification: TRON Meteor preamp TRON Voyager 20B SET power. Speakers: Avantgarde Duo. Digital: computing at last with Prism Sound Lyra 2 A2D converter