It has built in rear amps, so hook it up to any stereo receiver and get the extra channels. Tricky part is balancing the output between your receiver and the adapter ... I went through the basics here ...
Nice little box, even if it only has the basic decoders. Sansui's synth modes are quite tasty. Rather than go for OMG total surround sound, I suggest adding a set of good bookshelf speakers for the rears and just using them for ambiance ... That's how I work quad here ...
If you like what you hear, pop the cover and set it out in the hot sun for a couple days ... that's the best way I've found to get the smoke of ages outta the things.
Pretty straightforward ... just connect the QS500 to the receiver's tape loop using the front amp outputs. Front amp speakers are hooked up as normal, rear speakers go to the QS500 which has it's own built in amps. The two channel tape loop on the back of the QS500 replaces the one you lose on the Pioneer, and you can also hook a direct quad tape deck to the Sansui if you like.
DO read the section in my previous link on balancing the output though ... that can be a bit of a head scratcher first time around.
Forgot I had this ... here's the service manual ... enjoy!