I got a thoughtful email from Kirk Bayne who is a member of the Quadraphonic Quad community.
Kirk pointed out that with a standard stereo amplifier and 3 passive speakers, you can decode our flavor of “quad” to surprisingly good results. Of course, this is not as accurate as using QUARK or proper hardware decoders to decode quad but it works, it’s cheap and quite fun way to get (Listen to) Music in Space.
Backstory:
In 1982, Brian Eno released Ambient 4: On Land which was the final in his ambient series which began with the famous Music for Airport. On the back cover of the release he draws a diagram and explains how he “arrived at this system by accident, and I don’t really know why it works.”
Kirk said that what Eno discovered was actually very similar to one of the old quad format Dynaquad which somewhat accurately decodes the same QS/Regular Matrix encoding math we use in QUARK. This method is also known as the Hafler System.
Hafler published these ideas way back in 1970 (see page 22). The article is a good explanation of the Dynaquad/Hafler system.
According to Kirk, the Dynaquad system resurfaced in the early 1980s as an inexpensive way to decode Dolby Surround encoded (Beta HiFi, VHS HiFi) movie soundtracks and Kirk sent along a link to this PDF of a 1984 article which on page 63 has an article titled “Surround Sound for less than $5.”
At minimum, this is a fun experiment… and if Eno’s in, I’m in. Time to get the old speakers and amp out of storage!
Brian Eno – Ambient 4: On Land (back) with diagram. 
Brian Eno – Ambient 4: On Land (front)