KFI had a weaker signal than usual last night, so I got it nulled at just the right spot with my indoor loop and heard Latin music behind it. After trying to figure out which Mexican station, or possibly Spanish-language station in the U.S., this was, I finally tried an SDR in Florida on a hunch. And the music on 640 there matched. I then matched that to the audio on the Radio Progreso website.
I listened past 0800 UTC to make sure I wasn't imagining the match between what I heard under KFI, which really faded a bit after 0800, and the Radio Progreso audio heard on the Florida SDR. When Progreso played a radio soap opera in the 0700 UTC hour, it was nearly inaudible on my radio. But the Cuban music traveled well to my speaker before and after the soap opera.
I'm assuming this is the 50 kW Radio Progreso station on 640 (there are two), so the distance from me to that transmitter is just over 2,770 miles -- my longest MW catch to date.
UPDATE: I heard lively Cuban music again behind KFI at 0809 UTC on Nov. 21, and it again matched what the Key West SDR was receiving on 640 kHz.