DAB music quality is usually slightly lower than FM broadcasts. Se the discussion on this thread for more information:
http://www.theanswerb...4.html#answer-5870840
Then you've got to transfer that to cassette. Since most DAB radios only have an earphone socket (rather than dedicated phono output sockets), you'd need to keep the radio's volume setting quite low in order to avoid possible distortion. The quality of the sound recording, onto audio cassette, would then (in part) depend upon the quality of the recording equipment and the type of tape used. (e.g. using a chromium dioxide tape, rather than a ferric oxide one, together with suitable recording equipment could produce a reasonable quality recording).
However cassettes have never been able to produce (and retain) the highest qualities of recordings anyway. Broadcasters have played music from vinyl, from reel-to-reel tape, from CDs and from MP3s but never (unless it was the only recording available) from a cassette. It's just not possible to get 'broadcast quality' from one.
Chris