I believe that Quadraphonic sound never became viable because the improvement over stereo from a dimensionallity standpoint was never significant. The advantage of multiple speakers is only significant when direction is the goal of the sound. After the original novelty wore off(cannons fired from all sides in the 1812 Overture), listeners were more at ease with the fidelity and three-dimensional sound dynamic that is easily provided by stereo.If 360 degree sound emission is desired, a second set of speakers firing on the B channel achieves an equivalent effect without the need for Quadraphonic recording.

Directionality became more than a gimmick with Surround Sound: since sound was now integrated with pictures, the 3 dimensional movement,flow and focus of sound was essential. I disagree with Char that Surround Sound is less effective at low volume level than Quad. The major improvement with Surround is the ability to fine tune the sound through use of complimentary speaker sets, enabling the widest possible dynamic, or range of listening.Powered sub-woofers have further improved the dynamic, giving powerful bass even at low volume. And because the Sub contains it's own amplifier,all power from the primary receiver is still available for the rest of the speakers, maintaining brightness and clarity.