I only have the vinyl records and have never heard any of the CDs that were reissued.
I think a lot of times when tracks are reissued there is a desire on the part of the record company/producer/mastering engineer to make make them sound BETTER than the original. I also think that a lot of times the tapes they are working from are not the originals but the eq'd master from the mastering engineer which was done at the same time the record was cut and a lot of tapes from that era are NOT in good shape or were backup copies of a copy of a copy and they simply do not have the sound the original tapes have. I also believe that, as in most things, we remember what we want to remember and forget the problems so unless you have the originals to compare to I think we maybe remembering something that was really not there.
MTCW and FWIW
ps. I do have the original Simon and Garfunkel vinyls and the many different reissued versions of those records and depending on when they were done and by who they sound GREAT or REALLY BAD.
In one of the earliest releases you can hear ticks and pops and I have always thought that in a effort to get CDs our the door in the beginning they simply played the record and put it on CD. Some of the later reissues were done from the original session tapes and were remixed by their original engineers and these sound, to my ears, very good and very well balanced. There is a set that was released by a Japanese company and those CDs sound much better than almost any reissue I have heard.