answer:
The Columbia Journalism Review's Who Owns What? database is invaluable for this sort of question. Although I can't get it to spit out a list, you can go through and check manually on each company's ownership of different types of media.
I checked through the Cs and so far came up with CBS, Citadel Broadcasting, Clear Channel, Corus, Cox, and Cumulus as outfits that own more than 25 radio stations.
