Connect it to either A) the pre amp out/Pre amp In on a 2 channel reciever or Integrated Amplifier or B) the tape monitor jacks. Think of the QL600 as an add on 4 channel decoder/amp to convert a 2 channel setup into a 4 channel rig. You'll drive the rear speakers from the QL-600 and the front channels off your existing reciever/amp. You won't be abe to run the rear speakers off the same front 2 channel amp, but if you had a second amplifier you might be able to utilize that to drive the rear speakers at a higher power then what the QL-600 can deliver. I just don't remember if the QL-600 had an rear pre-amp out connections to do that. Its been a while since I've seen it.
I had one years ago, and used it quite a bit. If you don't have a preout/pre-in then connect it through the tape monitor loop and then the tape decks get connected to the QL-600's tape monitor loops. Either way I found that it worked best if you adjust the volume on the reciever to about mid way(12 o'clock) and then use the master volume on the QL-600 all the time. Just watch out if you bypass it since the volume will be pretty loud on the main system.
Sadly the 10 watt power output of the QL-600 just doesn't do justice on powerful front channel amps, and the SQ decoder was mediorce in my opinion. I gave it up for a Yamaha DSPE-300 Pro Logic Decoder.