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I have seen some old cassette tapes that have either dropouts or a significant loss of the high frequency end of the spectrum (no highs / treble). Sometimes, I have traced the problem back to something very simple and basic. Cassette tapes have a pressure pad made out of felt and held against the playback head with a phosphor-bronze flat spring. I have some old tapes where the pressure pad felt is missing (probably due to dried up glue) producing the mentioned problems (dropouts and/or no high frequencies). I have fixed this problem by sacrificing a donor cassette tape's pressure pad and installing it on the afflicted cassette tape with very good success. So, if you are having that sort of symptom when transferring old cassette tapes (I have some that are 50 years old), then check that out. You will find that the pressure pad holds the tape tightly against the record/playback head and yielding improved frequency response and less dropouts. I have used contact cement to hold the donor pressure pad onto the spring of the afflicted cassette.
Craig
Craig