Here in NE PA for several days, Boise, I had been getting a het on the high side of 660 while trying to hear WFAN as background on a paint job. It was late afternoon each time, the sort of power-up time for European hets. It didn't like a thousand cycles, though. It sounded lower-pitched ; maybe 800 or 900 cycles.
I idly checked, and evidently 661 is not a EU frequency anyway. So Castro is probably easing the regulations (if there are any) on AM signals. Or maybe he's engaging in some Cold War nostalgia just for old times' sake. Maybe one of his advisors told him that it'd be good to put on stations called 'Jammin 710' or 'Jammin' 660'.
710 and 660 are frequencies where there aren't too many domestic stations anyway. I don't know if this makes the frequency more or less susceptable to hets than, say, 1300 or 1340. The guess here is that the number of stations on a channel doesn't matter. But on 660 and 710, though, the field of suspects diminishes.
That's quite a poke -- Cubs to Idaho. What time of day was this?