Why so low?

At some point in my life, I decided that I had had enough Lincoln Mayorga.
Harry James was at the end of his career when that LP was made, but I am sure it has better musical content than did the typical Sheffield.

You will find this 'low' in many of the early CD's, especially those issued in the 80's and early nineties before compression took over. I have almost every Sheffield LP, my favorite is Amanda Broom, The Rose.

While compression undoubtedly plays a part, I believe that most of the reason for the low level is because direct-to-disc are mixed AND CUT in real time. The engineer running the lathe that is cutting the master has to guess what the loudest signal will be on the recording. With analog tape if you guess wrong, the level exceeds 0 db and you get a little tape saturation. But the cutting engineer has to set the inter-track distance before or while the master is being cut. If you set it too wide, the maximum length that can be recorded is less. If you set it too narrow and the level exceeds what you expected, the stylus cutting the groove modulates through the wall separating the current groove from the previous ... and you start over. For this reason, I think the engineer fudges a little to give themselves more room for error. Of course this also affects the S/N ratio on the disc.

I listen to classical orchestra recordings more than 90% of the time. I have approx. 3500 LPs and about 400 cds,again about 90% classical orchestra.
I have been listening to records for over 50 years and now have a $25K system.
I have learned not to make or accept absolute statements about pretty much anything, and certainly about sound reproduction.
But I am willing to say that I find the Sheffield direct-to-disc recordings of Wagner and Prokofiev, recorded in a studio (actually a cinema sound-stage), to sound more alive, present, grain-free than any other recordings, analog or digital, in my collection. This has been consistent over all the rooms and systems I have had since I first got these records back in 1980. Nothing else I have heard quite as convincingly captures both the tonality and the feel, the clean impact, of the instruments. The low brass, with great parts in both the Wagner and Prokofiev excerpts, are especially vibrant, clean, and alive. I have other records which fairly well capture the ring of brass instruments, a distinctive "brassiness" which enhalos everything from the pocket trumpet down to the Sousaphone; but on these discs there is a bit more authenticity to this "ring" than elsewhere. The violins, the glory of the orchestra, are somewhat bright and hard-edged; but the tonality, body, sheen, and glow of massed strings is amazing enough that it seems certain that the edginess is real and not artifactual--I am sure that's how they sounded in the studio.
The dynamics, as indicated above, are outstanding.

I also feel the Sheffield Harry James, Thelma Houston, and Tower of Power lps are outstanding in the same ways as the Classical stuff.
The Big Band lp is perversely reverberant though I guess that reflects the super-accurate way in which the direct-to-disc process captures the sense of the original space, in this case a high school gym!

The all too common lacunae involve the quality of the music.
The Wagner and Prokofiev are given average performances and personally I don't care for excerpts.
Harry James never was a very exciting trumpeter and he outdoes himself on these three beautifully recorded lps.
Thelma Houston is given a competent but uninspired group of studio musicians to work with. Even so, for my taste, "I Got the Music in Me," the opening number, with some powerful and gorgeous trumpets, and a great gospelly voice, rocks the rafters and provides some extra reward for owning the album (besides the great sound).
The Tower of Power album is probably the best of the lot, musically. As usual, the sound is extraordinarily alive and grainless and dynamic. The material consists of runthroughs of some of their hits. Not bad if a bit rehashed.

All in all, I agree wholeheartedly with the OP--They are a cut above.

(I haven't heard cd transfers of these records so I have no idea if their aliveness and thrilling tonal accuracy survives the big domain switch. Has anyone out there heard both?)

Also since they could not remaster if they overmodulated they kept a larger then normal margin of error below maximum volume
They are really excellant sounding recordings
Alan