The first, and possibly only, time I heard what could be understood as "true" quad sound was at an ELP concert. It was at a hockey arena, Maple Leaf Gardens, in Toronto, and we had cheap seats up in the grays. Turns out the ushers decided on some random basis that our particular seats were obstructed, and they put us on the "floor". There were speakers set up in the four corners of the rink, with some extra ones on the stage. In effect, we were inside the quad environment.

It was very cool at the time, the coolness not being at all hindered by chemical alterations to the synapses.

Although I owned some so-called quad vinyl, I never actually had the equipment to play it back properly. In theory it was "stereo compatible", although it always sounded kind of thin to me.