I remember it all too well. A buddy of mine had a reel to reel (and no, I'm not entirely sure what that is....) and could play the music backwards just by putting the reels on in reverse order.
We did all the classic satanic songs, especially, "Stairway to Heaven" and "Another One Bites The Dust".
I never could make out the "message" in Stairway to Heaven, but I always thought the message was pretty clear in the normal direction. Wasn't really sure why the backwards masking was necessary...
Another One Bites the Dust, however, was a different story. Backwards it said: "Decide to smoke marijuana, marijauana, marijuana"...So now I only listen to PRE-Dust Queen records.
The majority of the songs we listened to backwards to hear the message we either couldn't really make out, or could only make it out if we knew what we were supposed to be hearing.
Danny