I used to link two amplifiers to a single stereo source and simulate "quad" by connecting the negative terminals on one set of speakers together so that the second speaker pair emitted only the difference signal between the two stereo channels. Adjusting the volume of the difference speakers higher than the main stereo channels produced a pseudo-quad effect out of stereo material. I'd call it "poor man's quad," but since you needed two stereo systems to achieve it, that's not quite accurate.