As you, unduobtfully, know, most of the tran/rcv freq. antennas, for the centers, are placed on, at least one, dominant hilltop (we actually have three main sights). One that (that I know of) Sofia uses is at nearby mountain Vitosha, which is facing one of our sights (I would estimate the distance between them 80 - 100 NM). In that way, without any problem, you may listen to that kind of radio transmition.
The ability to make operational use from listening to adjacent atcs was new to me. This is not possible where I am because there is no way to tune a receiver to another than a preselected frequency AND to put the signal into the control room.