Cassette Tapes..Dolby B or C?

It has been my experience that using Dolby B or C takes more than noise away. It also dulls the overall sound. If you listen at higher volumes the Dolby helps to remove the hiss. But you mentioned "quiet background music". If listening to lower volume levels it may sound better to turn the Dolby off.

I agree, dolby off at low volumes, you be the judge at higher levels, B,C might take away fatigue.

Every prerecorded cassette that incorporated Dolby noise reduction should have a Dolby logo/marking on the tape or the box. As I recall there were many prerecorded cassettes that were made in Dolby B, but I never saw any prerecorded tapes in Dolby C. I personally recorded many cassettes in Dolby C from material off of CDs. I had excellent results, especially when using high-bias blank tapes.

To address your question, you should not assume that prerecorded tapes are Dolby B unless they are marked as such. In any case, if the playback sounds best to you with the Dolby B switched on, then go ahead and listen that way – you won’t hurt the tapes.

If the tape is pre-recorded and marked Dolby, it is Dolby B. Dolby C was available to consumers while recording their own tapes, but was not used on commercial tapes. As it has a more agressive EQ curve for encoding and decoding, it will make the prerecorde3d (Dolby B) tapes sound duller if used on playback.

Dolby HX Pro is a playback only processing and is not, to my recollection, usually selectable.

The tapes are plainly marked "Dolby" but no indication of B or C. I guess B is the right answer.