Unlike Dolby B, Dolby C is in my experience much more forgiving of playback without Dolby decoding.
For Dolby B the encoding/decoding was heavily biased on the "pre-emphasis/de-emphasis" of high frequencies, (but also dependant on the level). The result is that playback without decoding always sounds a bit wrong. With Dolby C, the bias seems to be more on the compression/decompression side, (but also frequency dependant). The result (in my experience) is that playback without decoding sounds a bit "flat" in terms of dynamics, and a bit overly bright, but not so "wrong". Tapes tend to loose top end with age, and the noise floor gradually rises, so old tapes that have been recorded with Dolby C may actually sound better if they are not decoded. If you know anyone that has a reasonable quality cassette playe it would be worth testing some of your tapes to see how they sound (after 10 years of ageing) with vs. without Dolby C. This will depend in part on the type/quality of tape that was used. (I have some recordings made with Dolby S on expensive metal tape that now sound terrible no matter what I play them on
Unfortunately with tape, high quality recording or playback cannot be done cheaply. The materials and manufacturing quality of components such as the tape heads make a big difference to the sound quality and cassette players of the quality of some of the old Nakamichi decks are just not available these days at any price.