They were invented in 1955, but high quality class D audio amplifiers have only been around since the mid90s, and are only now available as simultaneously good,

They were invented in 1955, but high quality class D audio amplifiers have only been around since the mid90s, and are only now available as simultaneously good, high-power, and cheap. Servo-controlled subwoofers are readily available these days; full-range servo systems are comparatively rare, because the end user generally wants room-calibrated equalization anyway.