I don’t know that I agree the sound is muffled because of age. I think the recording machine may have been a little too gentle on the correction, particularly if, as you say, you tried a bunch of players and they all did the same thing.
Cassettes are always a juggling act. Tape hiss and crisp, high music frequencies are always at each other’s throat and the purpose of all of the Dolby services is to give the music a head start. If getting into Dolby and getting back out again didn’t match, then corrections are always going to be a patch job.
Obviously, we’re not going to say “push this button and your troubles will be over,” so you need to give us a push. Post some of the muffled work on the forum. You can use WAV, but I think you can get more on with FLAC.
It may be that the heads and drive mech. need cleaning, the head azimuth may need adjusting, etc. the article in the Wiki addresses these issues.
Before I made my tape cassette conversions I sent my trusty old Nakamich BX-2 to Bowers&Wilkins (the official BAK repaira agents) for a full service - worth it I thought as many of the tapes had material on them that was irreplaceable and of value to me.