It just seems strange that an AM public radio station is successful, and I would have thought the music format might work there.Umm donor size? for WNYC-AM did you consider that? Even in Public radio the PD's and GM has to consider a format that justifies the maximum amount of donors to donate to a venue like WNYC and others. OK Probably WNYC could do what you want on an HD-2 or HD-3 feed but they are doing ok for now with news/talk format from NPR, PRI and local productions.
https://en.wikipedia.org/wiki/KPBS-FM
https://en.wikipedia.org/wiki/KXJZ