Of course, this is one of these eternal masters that pop up from time to time. I've read it in this forum, and for instance, it's a recurrent question in 78-L too...
We must also think that many of the latest 78s were not recorded on a 78 master directly by the artists, but were transcribed from masters recorded for LP/single or from tape. We should reformulate the question to
artists still alive whose recordings appeared on 78rpm format to be more precise...
At least the Spanish HMV pressings from 1955 onwards, from British masters, which have matrix numbers in the 78-7XCE- series, I believe were transcribed from 45/33 rpm masters. Horribly transcribed, by the way, with frequency limiters and strange EQ, I suppose to adapt them to lower range record players. Unhappy idea, for they usually sound terribly dull and muffled.
This is not the case with late British Parlophone issues, which were pressed at EMI Spain under the Odeon brand. Those in the late 204.000 series tend to sound full and vivid on modern record players.
I never understood why our Spanish EMI pressings of this late era, being all made at the same factory, have such enormous quality differences, Regal and Odeon (204.000) being systematically much better than La Voz De Su Amo or the 184.000 Odeon series pressings. Not in the pressing material, but the difference is in the transcription recording.
Even in the case of the many frequently dubbed matrices, which in Spain carried the suffix -T1 added to the matrix number, the dubbings are good, or sometimes very dull and unpleasant to listen to in modern equipment. It's clear to me that there must have been different technicians or teams with different techniques and probably different budgets for doing the work. It's something very frustrating in the Spanish Odeon records from 1950 onwards, of modern recordings, specially in the ones published in the 184.000 blue label series. For instance, I love the British Parlophone recordings by Roberto Inglez and the Savoy Hotel Orchestra. Many were published on Spanish Odeon, and the 184.000s are spoiled dubbings in many cases, while the few later ones in the 204.000 green label series are using the original matrices, or when dubbed, are much better done than the others. Maybe it's a matter of time, simply, the 184s being earlier (1950-1952) than the 204s (1953- onwards).
The original British Parlophone records, in the F (plum label) or R (black label) series are the thing to go for, of course. But these have to be imported from UK. There plum label Fs are made of poorer material than the black Rs. A matter of timing too? Are the Rs later than the Fs?