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.
Cassette Tapes..Dolby B or C?
I still have a tape deck in my system, and have a few tapes that are nice for quiet background music. The tape deck has a switch to select Dolby B or Dolby C (or none). There seems to be no marking on prerecorded tapes to indicate the type of Dolby processing. On a tape I was just playing B sounds about right. Should I assume that all prerecorded tapes are B unless otherwise stated?