I need one of those! Was that China Radio International by any chance? I listen to quite a lot of world news on that station and most of the correspondents and presenters speak perfect English with a hint of their having been taught by an American EFL teacher.
There's a chart showing what's available on various Short Wavebands all over the world
HERE and there's some fascinating material available.
Strangely, the Korean Broadcasting System is relayed from the Babcock International site in the tiny village of Skelton, in the North West of England. They use a 250KW transmitter, so should be loud and clear in Bulgaria. Skelton Mast is the tallest structure in the UK mainland.
Yeah, did I mention that I
need one of those Russian boxes? I have a lot of old Soviet radios in my collection, the technology is fascinating as they're generally extremely over-engineered and reliable but are around ten years behind the West in terms of component development.