In addition to vintage and modern (by Involve Audio or Dolby) hardware decoders, you can listen to these songs in quad using the “decode” function of QUARK by routing the audio from your playback browsers through your DAW using an application like Loopback or BlackHole.
A couple of people on the Quadraphonic Quad forum used JUCE and BlackHole to build a stand alone app. Franck made a great video based on instruction from BJMarsh. Many of you probably don't have Xcode on your computer so here is the already compiled app for download. Just use the instructions on the video starting here. You'll also need to "Allow Microphone Access" and you also may need to do this to convince your Mac to open the app.
Also, the Stereo Lab software (by Richard Brice) is excellent for decoding spatial audio music into discrete tracks. It does all kinds of other amazing tricks too. The only difference is this software does not process in real time.