UPDATE: May 2020
The temptation to acquire the new SONY (2020) box set of Gerhardt's Classic Film Scores proved too great to resist and I now have had the opportunity to compare the sound to previous issues.
I used just one disc from the box set for comparison purposes - The Lost Horizon. I chose this particular recording because I have on hand not only both previous RCA/SONY issues, but also the 2017 Dutton/Vocalion SACD.
I started by comparing the 2011 SONY remastering to the new box set, because I believe the new box set has not been newly remastered and is thus an exact reissue of the 2011 release. Going back and forth many times, track by track, I conclude that for all intents and purposes the sound is indeed the same. Transfer volume levels and dynamic range are identical. However, there were times when I thought I heard subtle differences. I had to go back and forth several times to determine if I was imagining it. And I can't definitively say for sure one way or the other. But I thought I could hear more inner-details here and there, plus slightly more potent organ pedal tones (on track 2) on the 2011 issue. Conversely, were the strings just slightly sweeter on the newer CD in the box? Moving upstairs to a different CD player with a headphone jack (using a good pair of Sennheiser headphones), I simply cannot say with certainty that there are any differences whatsoever.
Next I listened to Dutton's 2017 SACD release - in stereo SACD. Now I can say for certain I hear subtle improvements over the two SONY CDs. But not as much as I was expecting. The primary improvement heard on the SACD (again in 2-channel, not 4), is in the atmosphere and ambiance of the hall acoustic. It is better defined and more spacious. But in all other aspects, the sound is very, very close. This surprised me and might help explain why SONY did not attempt another remastering effort for the new box set. If even the extremely skilled technicians at Dutton could provide only a subtle improvement, at least in 2-channel mode, SONY can't be faulted for not taking another stab at it. The benefit of the Dutton SACD, of course, is the ability to hear it in 4-channel surround sound, as it was originally recorded.
Finally, I dusted off the original 1989 RCA "Dolby Surround" CD. Here is a completely different sound from all three newer reissues. And it is not subtle. It is more laid back, smoother, richer, darker, more bass-heavy and less detailed. Dynamic range seems compressed as well. I suspect the original master tapes were not utilized for these initial CD releases. It is the aural equivalent of a blanket having been thrown over the sound. Yet, it does smooth over most of the technical flaws in the recording itself, which was likely very beneficial to early CD playback capabilities of the time. It reminds me of those RCA Gold Seal reissues from the 90s, which tended to be smooth and...well... "golden", but not particularly detailed or dynamic.
To summarize, it is a nice convenience having all 12 discs gathered together in one box, and at such a reasonable price. It is a pity, however, SONY did not also include Gerhardt's Spectacular World of Classic Film Scores, and the two discs of John Williams's Star Wars trilogy (plus Close Encounters Suite). SONY did not reissue these in 2011 either, thus they've never been remastered after the initial 1989 CD release. (Note: Return of the Jedi was recorded digitally, in 1983.) This new box also has the significant drawback of not including a booklet or liner notes of any kind. But for those who don't already own the 2011 individual releases, this is self-recommending.
As a hopeless completest, I am keeping both collections on my shelves - along with the 1980's originals AND the Dutton SACDs, as they become available. (There have been 5 titles released thus far.) And it's worth noting, Dutton is harvesting individual tracks form the aforementioned Spectacular World album as fillers, when appropriate, for these SACDs. I hope they will resurrect the Star Wars recordings next, as they are the highlight of the entire series, both in performance and recorded sound.