Buk used a very basic cheap radio, if what I've seen in pictures is representative of what he listened to. Certainly in the earlier days he used whatever he could lay his hands on, but I sense that he didn't really care about the loud guts; he probably just liked the symphonic form in the background to ground his thoughts in an unobtrusive way. Any radio can do that; AM or FM be damned.
Here's a picture of his desk (from this site; thanks, mjp) with what I see as his typical radio. Probably has AM/FM, but little more: