I was one of those GI's (USAF, actually) in Germany in 1980-82 and buying up nice gear, but I can say that from my view, the Exchange had pretty run of the mill stuff that we could have easily found at any US stereo shop, poorly displayed. At Bitburg, we had an Audio/Photo Club that had much better stuff, properly displayed. I'm not sure I knew of anyone that bought at the exchange.
As to the winners, I know the love of 1970's Sansui, but to me, once the 80's hit they fell fast and hard. I thought pretty much all the top brands blew by them.