I have heard that 100 does bill better than many people think or see in the trades. My guess is because they do some very smart marketing events (concerts, Live on the Green, lots of local music venue remotes, online, etc.) They have taken a cue from the various country sideline (non-OTA) money generators that you have mentioned before.
100 markets to local players and wannabees, the ones who want to be part of the action. Mission statement in the office, to "serve the music industry." You have a local band, halfway decent. Not great, not going anywhere, but okay. Play their advertiser venues and get featured. From other side, say you have a business you want to hip up, they being an artist to come play a set. Throw in social media and its all circular, makes people think 100 is a big wheel. They sell w/out Nielson to hide the ratings but if you look at Mediabase numbers, maybe 1.2 listening in afternoon drive, rest is < 1k.
Curious about where the billing numbers come from.