It's a stereo amp + surround support, it's designed for 4 mains, and 2 rear surrounds.
The surrounds don't effect the main speakers.
It's an early surround type system, it's similar to the tweeters you get in cars in corner of dash. You can think of it as stereo surround, certain frequencies are sent to the surrounds. It works really well despite it's age.
For example, on Steve Mcqueen's Le Mann film, there is a section where he is looking at the track with only birds tweeting in background. Well those bird tweets I can hear on the surround speakers. I don't fully understand how Pioneer did this as it's taking only a 2 channel source, but somehow it works, whats more amazing is I can still listen to regular music and it all sounds good even with the surrounds still on. Pioneer were the most advanced in digital in this period from their work with laser disk in the 1980's.
Again I know my stereo system sounds odd, but it sounds totally fine. Everyone that listens to it says it's really good system. I even put the system into 2 speaker mode with them listening, or disable the surrounds, and everyone agrees with me it's the 4 mains + 2 surrounds thats bests.