Well, if you don't match your use of noise-reduction on playback to the use on record, you're getting something that does not correspond to the original sound at all. E.g., record with Dolby, playback without Dolby, result is artificial boost of the high-end. Record without Dolby, playback with Dolby: muffled top.
(And you need to match the Dolby mode: B versus C)
Though perhaps the artificial boost of the high end, when playing Dolby recordings without Dolby, helps compensate for the never-cleaned tape heads in your old car cassette player.
The real problem is that the analogue cassette was a substandard recording system. You can't make it sound good, you just have to pick what sort of ''wrong" you prefer. That's what Dolby is about, trying to offset the basic crappiness of the medium. In fact, pretty much all analogue media need some sort of technical fix to make them usable (yes: even vinyl: RIAA compensation).
dbx did a better job than Dolby, but never really caught on.
Fortunately, the analogue cassette is completely obsolete.