Hi Pete, I am no expert but a friend of mine had one of these many years ago and he loved to tinker with it. My uneducated opinion is that's about all it was good for. I don't believe it can be used as a standard integrated amp, but takes a tape out feed from an amp to power a second set of speakers. He had a main 2 channel amp and fed the "front" speakers with the main amp while passing the signal in parallel to the QS 500 through tape out from the main amp. The "rear" speakers were powered by the QS-500 fed through from main tape out. If I recall, it wasn't truly decoding a quad signal but had a number of options to simulate quad.
Again, I am no expert, just my recollection.