I have thousands of LP's from the 50's & 60's ... mostly jazz and classical. One thing I've noticed is that when tape hiss is present the highs are extended. On newly pressed records, some engineers "de-noise" the reissue to get rid of the tape hiss. Get rid of the tape hiss and the highs are eliminated as well. Therefore the " muffling" of the sound. Typical of this are the Mosaic reissues. I have several where I also have originals to compare them with. No contest ... the original wins in SQ every time.
Frank
Frank