I played my dad's Johnny Cash tape on a Walkman for 6 months before I caught the real problem
Back in high school I thought every cassette I owned sounded muddy and flat, so I just figured that was how tapes were. Then my buddy Dave popped my copy of Johnny Cash at San Quentin into his car deck in his mom's Civic and it sounded full and loud, night and day from mine. Turns out I had been storing all my tapes in a shoebox on the dashboard for months, and the heat had been slowly cooking the tape inside. My bad habit was the problem, not the tapes. How do you all store yours so the summer heat doesn't wreck them?
I did the same thing in my first car, left all my tapes on the dash through a whole summer and they got so warped they barely played. Now I keep them in a plastic bin in the house, never in the car unless I'm actually listening to them.